I have a weaker erection than i had before. and i can't keep it erected for a long time like i also can before. is this a cause of overmsturbation?

No, but...: Not unless you masturbate so often you forget to have normal sexual intercourse with a fertile partner. Masturbation is normal behavior, but don't rely on it so much that it interferes with you having a life.

Possibly: If you are masturbating, say, a dozen times in a day (in other words very frequently), then yes, after you have cum a few times and start again, each successive erection will become weaker. It takes a while to recuperate your vigor. If you are having weak erections that are not related to very frequent masturbation, go talk to your doctor about this.