Please help absolutely worried about my penis

[nonc]

Hi guys this morning i realised a yellowish bump on my penis which i thought was dirt. But upon cleaning i realised it was some sort of fluid filled sac.

I never had unprotected sex before and the last time i had sex was 25th of July.

Please give your opinions on what you think it is

*P.S

I just went down to the std clinic got tested for herpes, hiv and syphilis. All were negative