The changing seasons are messing with all of us in my house. Hard to sleep, early to rise.
My DD is 3.5 and when she started waking at 6:30 (way early for us), I said, "DD, you don't have to go back to sleep. But it's still dark out and the little hand isn't on the 7 yet. Can you play quietly in your room til the hand is on the 7?" This morning she was singing to herself when DH went to get breakfast, and he said good morning, they had a good chat and everybody got up.
Maybe give your DD suggestions for what to do instead. There's also a Calliou book about "bad dreams" that may help her if it's bad dreams. My DD will sometimes cry in her sleep - they have VERY vivid imaginations at this age. If she's crying hard, I go in and whisper, "Momma's here. It's okay." and pat her back. I talk to DD about how pictures in her head are not real and Mommy and Daddy will always take care of her.
If you think the urge to potty is waking her, can she go to the bathroom solo yet? If not, put a potty in an accessible place for her and tell her she can use it. We like the kandoo (or generic) wipes. I taught DD that she can use a clean one to wipe her hands, too.