Had a light fishy smell a few day before period, now have a rotten smell at the end of period that has little to no brownish blood and light mucus.

Bacterial vaginosis?: The most common discharge with "fishy" or "rotten" odor is bacterial vaginosis. BV is overgrowth of mostly normal bacteria. It's not an STD, but more common in women with STDs like gonorrhea and chlamydia. Trichomonas, an STD, can cause similar odor and discharge. Of course chance of STD depends on risk, partner history, etc. BV is easily treated. See a doctor soon for testing, treatment.