Look, I don’t know all your details or clinical history, I’m a healthcare professional and I have had some variant of that for a year now. I have been to the best professionals, did all the AB therapeutics and also tested negative for any infection. Even after that, and a year later I was deeply afected in some periods and always worried and anxious.
What I can tell you, by experience and research, is that what most of you have is the same thing, as I have/had, a somatic manifestation of anxiety and hypervigilance. There is no wicked, undetectable infection, if it tested negative, it’s negative.
Your subconscious is capable of manifesting physically in several ways, and that is one of them, for instance, excess production of smegma, that leads to irritation of the skin, maybe some marks and irregularities, or even small allergic reactions. Plus, even if you are generally healthy but keep looking for signs of an infection in your body, and comparing them to what you found on the internet, you will eventually find something similar and further convince yourself that you are infected, and from there it will only get worse. If a pimple or a small mark appears on your back or belly you will probably think nothing of it, or not even notice it, if it shows on yours penis or testicles you will go worried sick, right? Before you had the risk behaviour you could have something, even on your penis, and maybe not even notice it, but now you keep looking for something. Plus, AB treatments can unbalance genital flora, by killing most bacteria and cause balanitis, usually by fungus in those cases. Also, that can recur and be hard to go away, although is usually not very serious.
Bottomline, if you had a risk behaviour, go to your doctor and/or get tested. If it comes negative for everything, it is negative for everything, period. Also, infections like HIV are much harder to contract then you may think, go check statistics of transmission. Bacterial infections have usually very evident symptoms in men, like purulent discharge, that will constantly stain underware, evident smell (balanitis can also do that), other STDs have also very clear signs, plus you tested negative. Unless you experience evident and persistent pain, scarring or wounds, stop thinking about it, stop looking for symptoms, forget that red spot that has you worried sick, forget that different texture and don’t overvalue things. Live your life and try not to worry or think about it. If you want further information, or look into similar cases, check for chronic prostatitis forums or something like that, all the symptoms are there, a bit different each case.
Hope this helps to ease your mind 