Unexplained bruising under armpit

I have purplish bruising under my arm pit and certain areas of my body. It happened after exercising in gym, that night I became itchy and scratched and the next morning had bruising where I had scratched. It did not hurt nor change colors when pressed on. They are itchy only when I am on antibiotics and get a yeast infection. I have the one under my arm in pit area for almost two years w/o change. I have one spot on same side of body on my hip and one spot on my wrist.
I recently had an allergic reaction and everywhere I broke out in hive it turned into bruising spot. I have been to physician and they biopsy resulting in no cancer cell. So is this common to purpura? Answers please! Physicians have none for me!
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[Dr. Kokil Mathur]

Hi!
It is very difficult to find the cause of bruising on net. It is possible that allergic reactions are causing fragile capillaries to break. On the other hand if you are on oral contraceptives or contraceptive patch or depot shot, then these can also cause bruises.
If you are not on any medications like anticoagulants, blood thinners or birth control pills then this could be due to clotting disorders, low platelet count, fragile spider veins, deep vein thrombus, deficiency of Vitamin B12, folic acid, or Vit K, a liver disease or certain cancers. It could also be due to pressure urticaria. If you are on medications then you need to discuss that with your doctor.  Take care!