I have NSU now and I had a very similar problem the other day -- very strong pain in urethra when I got an erection. I, too, have been working with my doctor for 2 months to get rid of whatever I've got (though, in my case, my most recent flare up of NSU may be a new infection I picked up 3 weeks ago).
The urologist thinks its viral, but geez, 3 months! I'm at a loss. going to see my GP again and see what he thinks, any commmets are appreciated.
My quick guess would be NGU, but no reason to have pain without discharge. And never heard of anything that gives more pain with an erection.
Not really sure what this could be, maybe try a uriologist.