27 weeks pregnant and keep getting severe premenstral like cramps plz help?!?!?!?

its been happening for 4 weeks now, everytime i rang the out of hours doctors and nhs direct they didnt understand what it was, i was called an ambluance and rushed to hospital and was told i had a urine infection and now ive got rid of that, the pain still comes, when i get it sorry too gross u all out but i feel like im about to have the runs but i dont my bowles are fine it normally lasts a good few hours but it shoots up my back and everything... what should i do if doctors dont know what it could be and i havnt got a urine infection anymore! my babys heartbeat is fine when im in the pain, so is it doing any harm too the baby also i feel sick when it comes and i go all hot and flusterd the pain always wakes me up... has anyone else experianced this or have an awnser for it... thanks :)