Early labor signs?

Amanda
I'm 18+3 weeks. Yesterday I had pressure in my lower abdomen and pelvis. Today I had a random episode of diarrhea just once. I drank plenty of water and felt ok until later when I was contracting and having dull back pain. I called my doc he said im probably just dehydrated drink more water abd rest. I explained drank plenty. He's not worried but I still am.