could it be herpes even after tested?

Several months ago I noticed a small "pimple like" area at the base of the head of my penis.  I waited about a week to see it if would go away, it didn't so I went to the doctor to see if this could be an STD seeing as how I had unprotected sex about 3 days prior to this blister showing up.  The blister (or whatever it is) is painless and very small.  The doctor took a culter and gave me a blood test.  It came back showing that I didn't have herpes or any other STD.  About 5 days after seeing the doctor the blister went away and left no type of scaring or mark.

Anyway, about 2 days ago another blister showed up on the shaft of my penis, it has the same symtoms (symptoms) as before.  Could the doctor of made a mistake, could the tests have been wrong?  Does it sound like I have herpes?  Should I go back to the doctor?  

Also for my own piece of mind, could this blister be anything other than herpes?  I will get it check out regaurdless, but as I am sure you all can imagine i'm very worried.

Thank you for any help you can provide.

[abc789xyz]

i think that in order for blood tests to work properly you need to wait a varied amount of time after the infection so that your body can develop antibodies. i think for herpes its like 2-3 weeks after infection and for hiv its like 3 months so maybe you should get tested again. good luck!

[gracefromHHP]

lesion cultures have a high false negative rate. Since you've had a return of symptoms the best thing to do is to return to your provider for further evaluation.

grace