What Causes Early Periods?

I have been on birth control pills for 6 yrs now and I am not suppose to get my period til this Friday but I got it last Thursday. It started as spotting but then turned into a period and I still have it now. Usually my period only lasts like 4 days. I thought this was wierd and I and I'm starting to worry. I haven't missed any pills but I have been dieting and loosing so weight would that cause this to happen?