Mine used to occasionally when I was younger, even with masturbation. It was like a 'burning' type of symptom in my urethra. I actually paid cash and went to a urologist without a referral from M.D. (was a student at the time) and he looked inside my urethra (I guess for any fluid discharge) and said that 'it was nothing'. Since then I have had some kidney stones pass through, but this would happen with urination more than ejac. (along with other symptoms such as back pain, bloody urine, etc.) If this is happening after sex, sometimes condoms may irritate the urethra, or some lubricants can as well.
It is probably from the seminal fluids irritating your urethra. But I am not an M.D. so you probably see one to be cleared from STDs, urethritis, etc. Just another perspective to add to the above.
For me, the problem was occasional really and has not bothered me in the past 10 years or so.
good luck finding out!