Irregular period after plan b one step

I took the pill 10 days ago, which is when I had sex. I got my period about 3 days ago and it's a week early. So I didn't worry too much, because I suspected to get it early. However, I have been experiencing really bad cramps and bloating, I usually get very bad cramps but they don't last for more than two days. It's already my third day and I'm quite in a bit of pain, lower abdominal and lower back cramps. I'm hoping it's nothing, but I would really appreciate some advice!!