The top of my wrist hurts when i flex my wrist backwards or put weight on it. is it tendonitis?

Possibly: Wrist pain with flexion and extension can be a sign of many things. It is best to get an evaluation. You can start with your primary care physician. He or she may refer you to a rheumatologist if indicated.

COuld b: Could be tendinitis, ganglion, wrist ligament tear. Check it out with a hand surgeon.

Possibly: Dorsal hand-wrist pain aroused by stressing extensor function of wrist suggests either tendon and/ or myofascia injury. Limited mobility with an ace bandage would seem a reasonable employment. Such problems markedly improved in one to two days. If not, then a doctor should be seen; sooner than later if any redness, swelling, or heat, which are all suggestive of infection.