My cycle used to be 28 days, now it is 31. Is that unusual?

I used to get my period every 28 days like clockwork, but now it has changed to 31 days. Is that normal? I have always had a 28 day cycle since I was 11 years old. What causes it to change? I know that it is only a 3 day difference, but three days can be torture when you are trying to conceive. (By the way, I am 32 years old if that matters.)