CD20 and cramps?

Charity • Wife. Mom. Baby Boy💙 8/9/17. Baby Girl💗 due 8/8/19.
I am on CD20 and I am having what feels like period pains but AF is not due until 10/20. The pain is in my lower back and lower abdomen... Could AF be showing up 5 days early or what could be going on? I've read where implantation cramps are like a pricking or pulling (and I'm not sure how that feels so I'm not sure if that's what I'm feeling) sensation/pain and I don't know how else to describe the pain I'm feeling besides it feels like period pains? HELP!!