Unexplained bruising below ankle

@chawk
It could be a blood flow and fluid retention issue. You may want to see a podiatrist and/or vein/vascular specialist. You may need an ultrasound and/or brachial index test for peripheral artery disease.

@chawk, I agree with @dlydailyhope that you really need to see a podiatrist or other specialist have your foot checked. Here's some information I found on your question.

"What does bruising below the ankle mean?
Bruising and swelling is a sign that there is damage to a ligament, tendon, muscle, or bone in the ankle or foot. When there is damage to tissue, there may be a flood of fluids and blood in the area. This is part of the inflammatory response to the injury."
-- 5 Foot And Ankle Symptoms You Should Not Ignore
https://www.joionline.net/library/5-foot-and-ankle-symptoms-you-should-not-ignore/

Good point, @dlydailyhope! I've arranged to see my podiatrist next Monday, unless an opening arises earlier. My best guess is that, aided by Ambien, I feel asleep and banged up my ankle in a graceless attempt to navigate from the easy chair to the bedroom. Since my feet don't have a great deal of feeling, who knows what sort of good trouble I was up to before finally crawling into my good and proper bed!

Thanks for the suggestion, dildailyhope. I'm seeing my podiatrist Monday and my GP a week later. Meanwhile, I'm planning to take a few walks and turn off the news...