Cramps, yellowish brownish discharge, back aches, 2 days after period ended. could this have something to do with my yeast infection?

Not yeast. STD?: You might have yeast (how was it diagnosed?), but probably not the only problem. Vaginal yeast usually causes itching and only a little discharge (or none), usually white. Colored discharge and cramps strongly suggest a different infection, especially (at your age of 18) an STD like gonorrhea or chlamydia, maybe with pelvic inflammatory disease (tube infection). See a doctor ASAP.