Do you know what std's you were tested for? If you only had blood work, it might've only been hiv testing and maybe syphilis testing you had done. You also should be tested for herpes, chlamydia, gonorrhea and hepatitis.
You are able to pop these and get gunk out of them so odds are these are non-std related. Since it's reoccurred, I recommend being seen to see if you need antibiotics for a bacterial infection of the skin/hair follicles. You can go to a walk in clinic which is usually cheaper than a regular doctor office.