Hello i have had a hard lump appear almost overnight on my left wrist, i joined a gym on sunday and used light weights could this be why?

Ganglion Cyst: Most likely it is a cyst which is caused by excessive fluid from your wrist pushing out against the joint capsule. The cause of fluid is repeated movements of the wrist bones causing irritation. This should improve with rest but will reoccur with increased activity. Only way to completely get rid of it is to have surgery. If it doesn't hurt leave it alone if it bothers you see a ortho surgeon.

Ganglion Cyst L: Ganglion Cysts are caused by repetitive trauma - most likely the wt. lifting. These cysts can be painful but are not cancerous. Initial treatment is Ice/elevation of the wrist, decreasing the inciting activity, and NSAID treatment (ie Motrin 800mg three times daily for one week). If you are not getting better, follow up with a Sports Physician/Ortho. for a steroid injection in the cyst.