I have read your query. One obvious concern here would be that even you could be HIV
positive. Your history states that you have had much unprotected intercourse
with the girl. Therefore, the possibility is high. Probably the viral load
in you is not high enough to cause a positive test result or it is too early stage in the HIV infection.
Whatever be the reason, I suggest you to repeat the test at 3 months from now.
Another possibility is that the girl may not be HIV positive. It may be a false positive result. It is better for her to test again with more confirmatory tests like Western blot.
Meantime, practice safe sex
, use condoms.
It is possible that you get infected with HIV now and present symptoms on a later date.
If your partner is HIV positive for sure, you must always ensure protected sexual intercourse for a better health to both of you.
Hope this helps.