What going on???

Im 18 weeks pregnant tomorrow and also a young teen so im higher risk for preterm labour. today I woke up with hip pain and suddenly I got a cramp like pain all through my back, my stomach even tightened a little. Now It keeps happening every few hours and I'm nauseous and have diarrhea (which I know are signs of labour) i can't tell if it's getting stronger but it's bad enough to the point where I have to stop and focus on breathing. Im so scared of going into labour and losing my baby. What is this? And what do I do???