Why am I so thirsty all the time!!?

Today I drank a large sweet tea from Mcdonalds, 2 Large glasses of milk, a whole carton of orange juice, two more large glasses of milk, and a doctor pepper. This is just from 9am to 7pm. Tonight when I go to bed I'll probably drink about 3 large glasses of water through out the night. I don't know if this effects anything, but I am 31 weeks pregnant. Is there any reason i am this thirsty? My eyes have gone very bad lately and I gained a LOT of weight the few months before I got pregnant. My grandmother says to look into and get tested for diabetes. Is this likely at only 18 years old or is there another common reason?