Could I Be HIV Positive?

I have tested negative for HIV at 3 months and 7 months after exposure. What freaks me out is that my OBGYN said it could take up to 2 years to detect! Some say its only 6 months tho.....and if my OBGYN is wrong for saying that, then I'm scared they didn't do the tests right either (also because I'm on the medical card so they never give good service around here) and I got false negatives. Can anyone tell me the appropriate time limit for HIV detection and also if I should get tested elsewhere to make sure they didn't screw up the tests! Thank you!