Small lump that hurt to touch near my armpit

That doesn't sound like a tumor. The fact that it's very small and you can't even find it anymore is actually very reassuring.
Tumors don't just disappear within minutes, and they're noticeable even when you're not actively pressing on them.
Just keep an eye on the area. If it starts getting larger and begins affecting the appearance of your breast, then it would be worth paying attention to.
To me, this sounds much more like an inflamed hair follicle, an ingrown hair, a tiny cyst, or a small area of irritation from friction. It could even be a small pimple under the skin, yes, even within the tissue in that area.
Also, I know you have OCD, so try not to keep checking or pressing on it. Repeatedly feeling the area can irritate the tissue and cause more tenderness or swelling, which can make it seem like there's more going on than there actually is.

@ashlynnmae

Could it be an early tumor? Sore because it's pressing onto something?

@ehdog
That is not how early tumors present. Ever. They do not show up as tiny painful bumps that disappear when you try to find them again. Tumors are persistent and are easy to locate repeatedly in the same spot.
If you can't consistently find it, then no, that does not sound like a tumor.

@ashlynnmae If you weren't doing anything to cause this (straightening shelves, etc.), I would have it checked out. That's where our lymph nodes are. Have you have a cold recently, anything similar that might cause a swollen node? Peace of mind is important.

I see that you have concerns about various issues here in Mayo Connect, and anxiety about several physical things. Do you have a PCP or nurse practitioner you can trust? If so, you might set up a system where you can call/write them about your concerns, and let them decide if they want to examine you. Might this help? Maybe you already have? I'm not qualified to give medical advice, but as a person who gets nervous about my health I hope you can find some support/information from your providers.

ehdog, Many years ago, I experienced something similar. It was biopsied and turned out to be a cyst. Biopsies were different then. Perhaps now an ultra sound (no radiation) would give you some answers and remove unnecessary stress.
Fast forward to 2023 I had pain, in L breast followed by tomosynthesis. The results were negative, I was told to give up caffeine. I did; the pain disappeared. In 2024 annual mammo was negative; six months later in 2025 I found a lump and the next morning called my GYN. I assumed it was a cyst but had it checked out and it turned out to be IDCS. I tell you this not to alarm you but to let you know everyone's experience is different. If you have concerns follow up. Early detection is key. Some lesions are very slow growing and can be assessed periodically, and never need removal, others are more aggressive. The previous comments you have received are correct. Dense breasts can be tricky to diagnose, but it is my understanding from my own research that there is an over diagnoses of breast cancer and most doctors would prefer to err in the side of caution. Talk to your GYN and begin with the least aggressive but best diagnostic method to answer your question and yes, keep watch over the area. Best to you and good luck.

I agree with all the above comments - however my experience in the exact location your talking about. mine presented with itching for almost a year but could not feel anything, until i did. A very small lump that was painful that didn't go away once i felt it. During the itching phase my Dr told me not to worry and in hind site i wish i worried enough to get a 2nd opinion. Turned out to be IDCS. Chemo, surgery, and radiation took a year of my life (and more). Of course we all hope its nothing but if your worried - get it checked! Its worth if just for the peace of mind. wishing you the best

I never felt a small lump, however it was painful in that area. Mine turned out to be cancer 8mm Stage 1a, so my recommendation is if you ever have breast pain regardless if your feel a lump or not, get it checked out.