Hi Emma,
I have been having the same, I started with it before my BFP last about a week or two, it stopped for a while and was traded with constipation and last couple of days its returned!!!
I even woke in the middle of night last night to go, I was so worried because the pains kept coming and going and kept having to rush to the loo! I feel nauseous as well, this only started badly within the last few days, although I have had the odd day of nausea before this. I really don't wanna eat anything because ut just seems to make it worse! I do have IBS but I haven;t suffered with it for years, didn't even really rear its ugly head with my last pregnancy - I think I got off lightly!