My period arrived early a day or 2 after unprotected sex, after finishing the period and a few days in i can feel cramping on 1 side. what could it be?

A few possibilities.: I don't think your early period had anything to do with unprotected sex. However, unprotected sex (particularly w/o using a condom) can lead to STD's which can cause pelvic pain. Sex can introduce bacteria into the bladder causing a UTI. Occasionally, ovulation can cause some pain on the side that's releasing an egg. If this persists or worsens or new symptoms happen, see a doctor! Helpful?