@jaymew This is a very interesting question, one that often get asked in Minnesota around this time of year, when people visit the State Fair and walk for hours and miles on hard pavement. Most of these people do this once a year, at most, and when they see this the next day, they freak out. It even has a pet name here - it is known as "State Fair Rash" and people seek treatment for it every year.
Here is the word directly from one of our local dermatologists:
"A 52-year-old woman complains that she gets this red, very mildly pruritic rash on her lower legs every time she visits the cow barns at the Minnesota State Fair. It lasts for a few weeks. This has happened to her the past 3 years. She asked me if I thought she was allergic to cows. It seems to last for 3-4 weeks, and gradually fades away. Currently taking a statin and high blood pressure medication. No other health concerns. She noticed the rash the morning after she visited the Minnesota State Fair. She said she was at the Fair “for most of the day and did lots of walking.”
Hint: There seems to be a horizontal stripe of ‘normal’ skin in the middle of the patch…………
What's Your Diagnosis?
Diagnosis: Schamberg’s disease (a.k.a. Pigmented Purpuric Dermatitis)
The condition represents and extravasation of red blood cells into the skin with the rupture of capillaries in the skin, secondary to prolonged hydrostatic pressure (walking). I also see this commonly in the spring when people so for long walks or play 18 holes of golf. In her case, the hydrostatic pressure was from walking around the state fair all day long. The cows were, of course, red herrings.
Especially telling is the band of normal tissue horizontally across the middle of the patch, where the elastic pressure of her socks kept the capillaries from fracturing! An excellent example of the pathology of the condition."
Additional Information:
PERSISTENT PIGMENTED PURPURIC DERMATITIS DEFINITION
An inflammatory process, but not a vasculitis, that involves the legs especially with the purpuric macules and subtle papules (Schamberg’s disease), lichenoid papules (lichenoid purpura of Gougerot-Blum), and scaly papules (eczematid-like purpura of Doukas and Kapetanakis), all of which seem to be variants of the same basic pathologic process.

COURSE
In each of the expressions of persistent pigmented purpuric dermatitis, i.e., Schamberg’s, Gougerot-Blum, and Doukas and Kapetanakis, the lesions begin as purpuric macules, and, in the case of Schamberg’s disease, they may remain flat. In the other expressions of the process, purpuric macules become purpuric papules. The purpuric papules of the manifestation known for Doukas and Kapetanakis often become slightly scaly. The original purple color of lesions in all three expressions of the process changes in the course of weeks to yellowish and slowly, in months, to brown. New purpuric lesions may develop as older ones fade.

The course of the persistent pigmented purpuric dermatitides is highly variable. In some patients, the lesions last for only weeks or months, whereas in others the process persists for decades.

INTEGRATION: UNIFYING CONCEPT
The conditions named for Schamberg, Gougerot-Blum, and Doukas and Kapetanakis are morphologic variants of a single pathologic process, namely, persistent pigmented purpuric dermatitis. Those morphologic variants have in common a superficial perivascular and interstitial infiltrate of lymphocytes accompanied by extravasated erythrocytes in the upper part of the dermis. Early lesions of Schamberg’s disease have no other histopathologic findings. The clinical variant of Schamberg’s disease that consists of annular lesions is known as purpura annularis telangiectodes of Majocchi, and it is identical histopathologically to Schamberg’s disease.

In evolving lesions of the purpuric dermatitis of Gougerot-Blum, a bandlike infiltrate of lymphocytes fills much of the papillary dermis and, at times, obscures the dermoepidermal junction. In the purpuric dermatitis of Doukas and Kapetanakis, there is no lichenoid infiltrate of lymphocytes, but spongiosis is present in foci of an epidermis that is topped by small mounds of parakeratosis. As these expressions of persistent pigmented purpuric dermatitis evolve over months, the number of extravasated erythrocytes decreases and the number of siderophages increases. If siderophages are found in the upper part of the reticular dermis, it is a sign that the process has been present for years. As lesions of this condition fade, their color changes from purple to golden yellow, thus the designation lichen aureus.

THERAPY
No treatment is really satisfactory, but the lesions wane in time with only pigment as residuum. Systemic corticosteroids have been advocated as helpful in interrupting the process. Many of my patients have reported improvement with AmLactin ™ lotion, twice daily for 1-2 weeks. In cases like hers, I’d recommend compression stockings before the next fair visit as a preventive measure, as evidenced by the clear band from the pressure of the elastic band of her sock!

So, my guess is that this is what you are seeing - a dermatologist may be able to confirm it.

I get this every year on my first day at the fair, unless I wear knee-high light compression socks. This is usually not an option because the temperatures in late August are around 90F - so I just wear it as a badge of being a "fair rat" - and it usually fades in a month or so unless I am on one of my walking jags, in which case it persists until winter slows me down.
Sue