L&D nurse told me my due date didn't match my baby's size is my due date wrong?

went to L&D on monday for severe stomach pain, which were contractions and I didn't know, because I also had a bad stomach virus. Anyways they did EVERY test.. my blood pressure ekg's and two ultrasounds a abdomen and ob ultrasound. My blood pressure remained high and I have gestation diabetes, but I've been controlling it by diet since 28 weeks. The L&D nurse reviewed the sonogram and came in to tell me that my initial due date is wrong the baby should be due sooner, but this late in pregnant they can't change the due date. Is it because the size of my baby? or because she's more developed than they thought? how can they tell when my due date is and should they have told me how far along I really was, If I'm like 37 weeks I'd like to know so I don't all of a sudden go into labor and think it's premature. Has this happened to anyone else? Also since about 20 weeks I cry REALLY BAD on and off... I cried so hard talkin to my mom about my husband until i didnt even kno y i was crying.