Hello thanks for your question, I feel sorry that there were inexperienced person at blood donation camp There could be two reason for blood clot under your skin during blood donation. No.1,while cannulating your vein the punctured the vein by a big size needle( as big size needles are used during donation camp) and the blood collected along the side of vein under your skin which later on formed clot No.2.while taking out the needle at completion of donation, the blood leaked into peri vascular space under skin,through punctured vein they didn t instructed you to keep pressure over the puncture sit for at least 5 to 7 minutes. Your 2nd problems, your getting pain while extending your arm,this is most probably due to thromobophilibitis of the cannulated vein.It will get resolve in a week time,if you have associated fever with it you can take antibiotic.Locally you can apply heparin ointment. Hope i answered your question. Get well soon Keep donating the blood to save lives.