Due for my period but instead i'm 3 days into only small spotting. is this normal &what might explain this? not on bc and not pregant.

Missed period: If you are sexually active and not on Birth Control, the most likely cause would be pregnancy, although you could have hormonal imbalance.

Period late: When one is "late" for a period, and is "certain" there is no chance for pregnancy, there can be a failure for the ovarian cyst (follicle) to rupture to release the egg and although spotting may occur, hormone levels are sustained and a true period may not occur until it ruptures. This can cause one sided pain, but it will resolve when the cyst ruptures. See your healthcare professional for tests.