Those last couple of weeks are such a tease, aren't they? With both of my kids I started feeling that constant pressure and "have to poo" feeling around 37–38 weeks. For me it was just the baby's head settling lower into my pelvis and pressing on my bowel and nerves. I'd sit on the toilet and nothing would happen but the pressure would ease for a bit.
Mild cramps, back ache and pelvic pressure can definitely be part of the body gearing up for labour, but active labour doesn't usually kick in until the cramps become regular, stronger and start to form a pattern. Some women have these pre-labour sensations for days or even weeks before the real thing. When my contractions started for real there was no mistaking it, they came in waves and got closer together no matter what I was doing.
If you're worried or the pain changes, give your midwife or doctor a call. They'll probably tell you to rest, stay hydrated and watch for your waters breaking or contractions you can't talk through. But what you're describing sounds very familiar to me and a lot of mums I know. Hang in there, not long to go now!