3days cramping and dark brown discharge when wiping, mucous like n very little red blood. Period 1wk ago n protected sex 1x since then. What is it?

Breakthrough. This sounds like a hormone imbalance or breakthrough bleeding. Sometimes this occurs because estrogen and Progesterone (your 2 female hormones produced in ovary) are not in the right balance to each other. Sometimes it's due to pituitary or thyroid issues. And lastly, sometimes due to weight (too thin/heavy). If bothers you can consider birth control pills to help regulate. Best of luck.
Midcycle. Sometimes you can have spotting with ovulation - which could be this, or it could be cervical - if it continues would get checked by your physician.