Diarrhoea is often consistent with onset of labour, the regular lower pelvic pains on their own could easily be explained by pre-labour, but the two combined sound like it's a good chance it could be labour. At 33 weeks I don't know that start of labour would necessarily coincide with a slowing of baby's movements, so I'd be getting checked out ASAP if I were you.

Personally I'd be more inclined to go straight into hospital rather than see your GP because if bubs is on its way a bit early they'll be better prepared to look after a premmie if need be. If you are in early labour your GP is just going to send you straight to hospital anyway. How far away from the hospital are you?