I have history of pre term labor currently 18 weeks pg. Last night had cramping, diarrhea and back pain. Rested now just back pain. Should I call dr?

Definitely. I always much rather have my patients call me with concerns early on rather than wait or worry about something that we could have dealt with with a simple conversation or something that we can prevent from becoming a problem.
Yes. Although your symptoms may be associated with premature labor, they aren't necessarily. They could be related to stomach flu which could cause dehydration. Or a more serious infection. So calling your doctor would help you.