call your doctor tomorrow. Tell him what happened. Ask for a prescription for Combivir or another prophylactic antiretroviral medication.
I am a nurse. When healthcare personel are stuck by needles, or exposed to a patient's blood, they immediately begin this type of medication. It will prevent you from seroconversion...essentially, it is your best defense to prevent you from becoming HIV positive (that is....of course....IF the other guy was HIV positive as well). This medication regimen is standard in all hospitals for healthcare workers who are exposed to blood via a cut or needle stick or whatever.
Just FYI, I had to take this medication regimen once in the past, I was stuck by a needle in the Operating room. Thankfully, the patient's blood was tested and he was HIV negative, and all is well. But, the few days I was on this medication, I felt like shit. Stomach was upset, and just felt like crap in general. It is not fun. But it is the only option you have to protect yourself after an exposure like this.
P.S. You have 48 hours to start this medication. It's about 70 - 90% effective, and is your best option at this point. Don't hesitate, call your doctor in the morning.
I hope everything turns out well for you

Take care.