Welcome to the forum.
The combination tests for both HIV antibody and p24 antigen are virtually 100% reliable any time more than 4 weeks after the last possible HIV exposure. Your negative result at 33 days proves you didn't catch HIV. If this is your only potential risk, then you indeed can safely have unprotected sex with your girlfriend. (That reassurance, of course, counts only for HIV and only for this particular exposure. I have no way of knowing whether you might otherwise be carrying an STD like herpes, HPV, chlamydia, etc.)
Here are links to two thread that goes into more detail about HIV test reliability at various intervals after exposure, and about the antibody/antigen tests:
http://www.medhelp.org/posts/HIV-Prevention/-A-Question-on-Testing/show/1347755
ttp://
www.medhelp.org/posts/HIV-Prevention/Need-your-help/show/1345664
So all is well. I hope this helps. Best wishes-- HHH, MD