The only adult risks for HIV are:
1) having unprotected, penetrative vaginal or anal sex with someone of unknown status, or
2) sharing intravenous drug needles
If you do not do either of those activities, you will NEVER have to worry about HIV.
Since your girlfriend has tested negative and isn't having sex with anyone else, you can rule out HIV. Not sure why you picked cancer as the only other alternative, as there are myriad mostly minor issues that could cause a swollen lymph node and knee pain. The fact that antibiotics have reduced your lymph node swelling points to a bacterial infection for that problem. Knee pain? See an orthopedic doctor.