Nope, 4 months is a reasonable time to be going out. You should be able to openly ask her anything that is on your mind
Don't feel bad that she didn't take it too well. Maybe she has some issue regarding her past (Don't want to put ideas into your head here, but perhaps something bad happened to her).
Just reassure her that she can talk to you about anything. At least you tried and made the effort.
Failing that, finish with her and find someone else unless you can deal with it.