Sexually Transmitted Diseases

[helpme]

My girl friend of 9 months tested positive for Clamydia 6 months into our relationship after getting checked for a vaginal discharge and odor. I was tested for Clamydia but my test was negative. We both were treated for Clamydia She developed the symptoms again after treatment. She was tested again and was told she has tested positive for trichomoniasis. Not sure whats going on with all this but If we have only been sexually active for 9 months with only one another how is this happening and why does my test come back negative. She also tested positive for Clamydia once before we met but she was treated. I have not been tested for Trich yet.
1 How is this happening?
2 Are both STD's detected by the same test ?
3 Can either of us be carrying any of the STD's from past relationship's ?
4 Is there any way either of us contracted it some other way ?