I have been on my period for 3 weeks what could be wrong?

Too long: You may have other issues that ned to be looked at, you should contact your gyn for an exam.

Bleeding: Hard to tell what caused it. Rule out pregnancy first. If pregnancy is negative, other causes could be anovulation, which had preceded the bleeding. A ruptured ovarian cyst can do it, etc. You will need history & physical to begin with.