You can still have an STD. See your doctor and get tested.
No sex with your girlfriend until you are cleared, That means for HIV too.
The testing protocol for HIV is:
1. You get tested at exposure to determine if you already had HIV.
2. Test at 30 days.
3. Test at 60 days. If this test is negative then it is very unlikely you have HIV.
4. Test at 90 days. This confirms the finding.
You'll have some explaining to do to your girlfriend as to why you won't have sex with her for three months but it must be done. You wouldn't want to possibly infect her would you?