Late period after stopping birth control

I stopped talking my birth control on December 8th. That was the last day of the placebo pill. Also the last day of my period. I was supposed to get my next period on Sunday December 30th. It always on schedule. I'm 4 days late now. Is this common when stopping birth control? I havent had any spotting or anything. I feel crampy every once in a while, like I have my period but I don't. Am I late because I stopped the birth control or could I be pregnant? Thoughts?