What could cause a change in hormones?

I’ve had my period 3 times this month, lasting 4–5 days each. I’ve always had regular periods; ever since I started menstruating at 13 I’ve had it every 26 days, never late and never early. My mood has also been kinda odd. I have depression but I’m on antidepressants so usually my mood is pretty level. Lately I’ve been so irritable, I’m snapping at everyone. Or crying during car commercials. I’ll eventually make an appointment to see my doctor, but any ideas on what causes a change in hormones?