22female. My period has been acting up for probably the past 6-12months... It's always 3days to a week, if not more, late. What could cause this?

Several causes. There are multiple causes for the problem that you are having, that will require a live assessment by a local GYN MD.
Menstrual cycle. are you getting cycles every month duration of cycles could vary from 3-7 days as several factors affect cycle length, such as stress , hormonal changes , medicines as long as cycles are not heavy and not very irregular there is no need to worry about it if too prolonged and heavy its advisable to follow up with regular PCP for annual pap and pelvic exam along with a possible ulttrasound.
See below. Unusual menstrual cycles don't necessarily mean that there's anything wrong physically. The menstrual cycle is under very delicate hormonal control, which can be influenced by things such as exercise, rapid weight change, emotional stress etc. But to be safe: see your GYN if you continue to have abnormal bleeding. SEE http://cle.clinic/1IDXMvq.