I'm seconding the others' suggestions to get a vet, pronto. I had a Pseudomonas infection in my mare's legs and used over-the-counter stuff and antibiotics prescribed by my vet (before he knew it was Pseudomonas). It turned out that the Pseudomonas was resistant to all antibiotics except 2 (2!!!) and was running rampant. We ended up having to start with IVs antibiotics. If we had cultured/biopsied first, we might have avoided some of that. Also, picking at it or rubbing it made it flare up and spread like wildfire. I would do as little of that as possibe.
In the end, we had a repeat flare-up and I used Vetericyn which knocked it in its tracks....but you need to use the Vetericyn soon, before you've got an infection that's resistant.