I had unprotected sex a week ago, 3 days later I started spotting and cramping, I've also been very tired & hungry. What could this be?

Nonspecific. Without further information, nailing down the precise answer may not be possible. Of course, unprotected sex always risks pregnancy. That being said, simply being 19 and stredded is enough for spotting, cramping, and hunger may depend on where you are in your cycle. None of this excludes pregnancy, but none proves it either.