It's been 9 weeks since I gave birth (vaginal birth). I have stopped bleeding and recently it started again. It's pretty heavy bright red blood. Just today I started to feel a little week. My sister says it's probably my period, but I am not sure. Should

Probably your period. The bleeding is probably your period, however, you should consider being seen by your OB physician so he/she can make sure the heavy flow is not from several more serious possibilities.
Probably. It probably is your cycle, but if it is very heavy and you are getting weak, you may have lost a lot of blood. I would recommend seeing your doctor for evaluation.