Thanks for writing to the forum!
If you are not on any medications like anticoagulants, blood thinners or birth control pills then this could be due to clotting disorders, fragile spider veins (strong possibility), 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 (this is a strong possibility in your case as it happens on one arm). If you are on medications then you need to discuss that with your doctor.
You need a comprehensive investigation taking all these points in consideration. It is difficult to comment beyond this without examining. Please discuss this with your doctor and get yourself investigated accordingly. Do let me know if there is any thing else you want to know. Take care!
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