How come I got really sick from an STD but my girlfriend didn't?

Did she see a doc? It is common for men to show symptoms of an std such as chlamydia, but for women to be completely asymptomatic. Be sure that she saw a doctor and had a complete std screen and physical exam, so that if she did have an infection it could be treated.
Maybe she gave it 2 . Maybe you got, it from her and she already had the acute symptoms. Both of you should be treatedat the same time to prevent pingponging the std back and forth between you.