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, 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 any of the above medications then you need to discuss that with your doctor.
You need a comprehensive investigation taking all these points in consideration.