FTM here - I had mild cramps for three days, and finally went to the Dr. after I started getting mild back discomfort as well. Turns out I was having BH contractions! Not painful at all, but just... crampy. Then, while I was being monitored, the pain sorta ramped up into "zing-iness". Again, mostly not painful, but not comfortable. They lasted for 30 seconds or more, wouldn't really "end", but the feeling would fade away. I've also had short, stabbing pain (and some "lightning crotch"), but they feel very different.
I never would have guessed that the initial feelings were contractions if I hadn't been hooked up to monitors and the nurses told me. Towards the end I could understand what the contractions were and how they were different from other random feelings down there. I completely understand that the real deal will be super obvious, and much more painful, but I'm sorta glad to know my body is practicing.