Re-occuring Blister type bumps on penis

[jsilver]

Hello, I'm 30 years old and masturbate atleast once a day. About 4 months ago I noticed a small red bump on my penis after masturbation. I figured it was probably some irritation due to friction. It went down on it's own but never completely disappeared.  Upon closer inspection it's actually a couple of small red blister type bumps that seem to keep getting irritated and coming back after masturbation. If I don't masturbate for a couple days it seems to go away but always comes back after masturbation again in the same spot. It is on the shaft, on the side of the penis. This bump is also itchy and slightly painful to the touch. I had a full panel of STD tests and everything negative.  I would ignore it but it is painful and unsightly when it gets irritated.  Any ideas of what this may be? I haven't really seen anything like this exactly on the forums-
Summary:
Red bumps keep coming back in the exact same place after masturbation
Looks like 2 small blisters close together
No visible puss or whitness- just red and possible small amount clear fluid-
Itchy and painful to the touch

Thanks
-JS