I just had my period then a week later it came back and started again. this has never happened before. what can this be?

Breakthrough : this is a common occurrence in the menstrual cycle. All too often I will hear women complain of having had a normal cycle and then also having episodes of bleeding at any point between those two periods. the top of fenders p I would recommend keeping a calendar and keep track if this is an occurrence for at least the next 2 cycles. if it continues you may want to seek advice from a local OB GYN

Lots of things.: Always rule out pregnancy first. However, many things can affect the timing and regularity of one's cycle (e.g. Weight changes, diet, stress, etc..). Don't stress. Keep an eye on it for now. If the changes are persistent for more than 5-6 cycles, see your ob/gyn.