I used to get cramping and a feeling like my period was about to come between the 4-6 week. I got the same feeling a few days ago when my period (if I wasn't pregnant) would have been due. Not fun, but as long as you aren't actually getting AF, you should be fine. Some people have spotting but turns out light spotting should be ok. So that was my rule of thumb. I even had cramping after my ultrasound, but turns out that is supposed to be normal.