Hun, I get cramps when I need to do a BM all the time normally!
Stop stressing, it is normal for your body to do a clean out before baby comes, especially as he/she descends further into your pelvis.
It is great to keep as regular as possible in these last few weeks, because after birth it hurts!!

In hospital, eat kiwi fruit with the skin on. Something in the skin is really good for it and I know people who have said it works better than fibregel (or whatever it's called)