Last night my dream was about a girl with black hair. She was crying, so I went to help her
Sounds like this girl is you and you are trying to help yourself but it hurts when you try.
Dreams are an expression of our inner thoughts, feelings, fears, hopes and desires. Nightmares are those same feelings and fears gone to the extreme. Sometimes there are physical causes, sometimes there are emotional.
I'm curious, but is there a similar theme to the bad dreams/nightmares? For example (I'm not saying this is happening but just for instance) is someone chasing you, or do you fall? If there is some general consistency, it is the mind's way of pointing at an area that needs to be addressed.
Rule out the physical if you can, but stay open to the idea that the nightmares could something that you have repressed in to your subconscious and it is coming out in your dream state.
Chris