Cramping question?

I just got my BFP a couple days ago and only missed my period today. I have been cramping since 5 dpo. I thought I was out this month. Does anyone know what causes cramping and lower back pain(cramping almost) during early pregnancy? I have been pregnant 6 times, 4 babies and 2 miscarriages, and I have never had this before. Any of u ladies have any insight on this? I'm still taking tests and they are getting darker so that's a good sign right?