Not sure if i'm pregnant!! HELP! Was on bc but stopped after last pill pack! Whats going on???

I've been taking aviane birth control pills for the past 3 1/2 to 4 months. Recently I decided during my last pack I wanted to get off of them because they were making me fat. My period works like clockwork and is always on time, especially after taking the pills. During the "sugar pills" (the green ones) I had my period which was on the 14th of april then stopped taking the pills on the 17th. (3 days later). It has been 15 days since I have taken the pills. My sister did the same thing had her period like clockwork then took her last pill and didnt buy another pack and got her period AGAIN like three days after she stopped taking the pills. Could I be pregnant??? Or would my period just resume like usual in the 28 day cycle like clockwork?