Why do I wake up feeling really hungry? Last night I didn't eat a big meal or anything and on days I eat big meals it happens too.

See MD for Changes. While it is normal for people to experience brief changes in appetite, a persistent increase or decrease in appetite shoud get you over to have a proper evaluation by your physician. Common causes of altered appeitite include anxiety and depression, pregnancy, thyroid problems, and more severe illnesses including infections and even tumors. Please contact your physician.
Metabolism. Try paying attention to your protein intake. Eating too many simple even complex carbs before bedtime can cause a hunger elevation over night, ...People who eat enough daytime protein at 3-4 hour intervals have higher daily metabolism and therefore less middle of night hunger, ...