Worried.

I just found out I'm pregnant yesterday, when I tested I got a couple super bfp. But I wasn't supposed to start my period for another week. Im only 3w5d. And pretty much all day today I've been off and on cramping pretty bad. This is my first pregnancy and I guess I'm just super freaked out right now. I honestly feel like I'm getting ready to start my period, I always feel like this a day or 2 before AF shows up. What are the chances this is a chemical pregnancy??