Urination Discomfort. What is going on.

Sometimes if I urinate soon after ejaculation, I will feel pain in my urethra near the tip of the penis. Also, sometimes I will feel mild discomfort even before I have urinated. The burning pain during urination usually only lasts for about 30 minutes with mild discomfort lasting another 30 minutes or so. During this time, it feels like I can't empty my bladder and I end up visiting the bathroom many times, only urinating a little at a time. This only happens periodically and can happen after masturbation or intercourse. I have had this problem for several years now (even before I became sexually active). Since it only happens once in a while, I haven't mentioned it to a doctor, but I'd like to know what the problem is and if I can do anything about it. I am hoping someone here can help me understand what is going on.