Just remember that a negative HIV test this quickly after the act is not 100% You can still seroconvert up to 60 days after exposure. Other things to look for include discharge from the penis, burning upon urination, lesions on the penis (either painful or painless), or unusual growths on the penis. If you exhibit any of these signs you need to see your doctor immediately. If money is an issue you can test at your local health department. They will be equipped to test you for HIV, gonorrhea, syphillis and chlamydia. Some can test you for herpes and/or HPV. Gonorrhea, syphillis and chlamydia are readily treated. Herpes and HPV can be treated for symptoms but you will carry them forever. Treatment for HIV has advanced but it would be important that you get evaluated medically as quickly as possible.
Now, that said, it is just as likely that you have not been exposed to these infections so chill out. If you have any questions feel free to drop me a PM. Oh, and next time wear a condom or abstain.
I've been drinking muddy water and sleeping in a hollow log.