I wouldn't be too concerned. Our bras can trap a lot of heat/sweat and cause a lot of friction making us prone to irritation and redness. It's common for women to get breakouts in this area. Popping them-- as you know-- doesn't help and can cause the bacteria to spread. Putting a little anti-bacterial cream or aloe vera on the spot should help it heal. Cover it with a band-aid if it still continues to rub against your bra.
With that said, just because you've had a precancerous birthmark removed, I would get a doc to look at it to be sure. My guess is it's nothing serious but it never hurts to get checked.
All the best,
It can be due to a boil, heat rash, sweat dermatitis or shingles rash if there are any accompanying painful blisters. For boils, Apply warm compresses on the boil and keep the area clean.Drainage of the boil is done only when it becomes soft and forms a head. Take some over the counter pain reliever and consult a doctor for examination and antibiotics prescription.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.