21 f period last month- normal &10 days after period ended i started bleeding again moderately. pain in mid back, fatigued. not supposed to start yet.

Bleeding day 15 ?: If your periods are normally 28 days apart, and last for 5 days, "10 days after ending" would be day 15, and if ovulation on this occasion occurred with two eggs instead of just the usual "one", there was a higher estrogen level prior to ovulation, and the drop which occurs before ovulation can then be similar to a "light period". This does not happen often.