Had a good anatomy scan but still having cramping?

Gabriela • First time mom. 23. Due 06.26.17
Iv had cramping on and off through out my whole pregnancy. I'm 20 weeks. I feel sometimes as though the cramping is getting worse and worse. Iv been to the doctor and he has checked my cervix and the length and everything is fine. I also had my anatomy scan on Friday and they said everything looked good. So why am I having this really bad cramping still? 
Is this normal, or is their something else they should test? 
I am going to go back to the doctor on Monday, but I honestly don't know what else they could do. I am not bleeding, just cramping and a weired tightening sensation on my lower abdominal. I am a first time mom.