Is it normal to have a period for 3 weeks and then not have another at all after that while on depo-provera provera (medroxyprogesterone)?

Yes: Bleeding as you descrbe is often seen with depot progesterone; however, should resolve after 2-3 injections at 3 month intervals. Call you GYN if symptoms persist.

Yes: Depo Provera (medroxyprogesterone) has been known to causes very irregular periods. Granted you have been consistent with the shot and not missed a dosage this should slowly correct itself on its own.