What Causes Missed Periods And Brown Vaginal Discharge After Bowel Movement?

Hi, i missed my period last month but i have not been having sex. I also started exercising, and slightly changing my diet. Also, I'm starting to have stomach pains and slightly nauseous occasionally. I have also experienced a brown and sometimes red discharge from my vaginal area every time i try to have a bowel movement. I'm very scared, and would like to know what's going on. I am a 20 year old and i weigh about 120, and I'm 5'3. I've never experienced this before.