No. It is very possible that she had it before meeting you and you both did not show symptoms. Or in fact they were mistaken for other things as you have mentioned. It is possible that the bleeding was a combination of both. Chlamydia affects the cervix and so does birth control. Honestly, the symptoms of any infection are general and characteristic to each individual one. How it affects the person who is its host can differ. I got bleeding right away when i had it last time. I also had abdominal pain. I was fortunate enough to get symptoms and have it treated right away. Up to 75% of women do not show symptoms.
I would not assume your girlfriend has cheated. Without her saying that she did, there will be no way to tell when she got infected. Both of you have been treated. So, go back in about 3-4 weeks for a test for cure. In the mean time do not have sex until at least 7 days have passed and then make sure you use condoms until you are absolutely sure the both of you have been cured. Good luck!
Thanks for the help and quick response.... Much appreciated