Did you receive oral? If you did, it's possible you have NGU - I'm assuming you're male as well. If you aren't a male, it could be bacterial vaginosis.
When you see this discharge, is it happening on it's own, or are you squeezing your penis to get it? If you have to squeeze your penis to see it, it's not an STD discharge.
With back pain, you might also have a urinary tract infection. Have you see a doctor?