I don't know what your problem is, but as said not all drugs work the same for all people. Some will find relief, while others will find things get worse. There are many SSRI's available, so there's many choices still available.
Perhaps try some other doctors and see what they say of your problem.
If you're not depressed/anxious and you only have this problem after orgasm, I personally would not want to take an SSRI. Unless of course it was the only thing that helped.
If it truly is a serotonin problem then you can treat that other ways. SSRI's usually have side effects and withdrawal effects if you stop taking them, and not exactly something people should take if they don't need it. Look into natural ways of increasing serotonin through diet and exercise. A couple supplements you could take would be Omega 3 and 5-htp. 5-htp helps increase serotonin in your body, while Omega 3 is an essential (meaning your body needs it and it cannot create it) fatty acid that is good for many things, a healthy brain and mood being just a couple. Protein would provide you with tryptophan which would lead to 5-htp and serotonin.
I'm not telling you to go against doctors orders. I'm just thinking that if this problem only comes after orgasm that you have time to try healthier and less dependent methods first, because you can avoid the symptoms by refraining from ejaculation. Plus, once you're on the SSRI it could be difficult to get off of it if you wanted to try something else later.
You might also want to just take some general supplements so you're healthy and your body has what it needs. What you lose in ejaculation is replenishable, but it still needs to be replenished. A multi-vitamin and multi-mineral will cover most bases. Extra calcium is good and depending on your multi-mineral a little extra zinc is good for male and sexual health. Don't exceed 50mg per day of Zinc. That's all your zinc intake combined, so food, plus multi-mineral, plus zinc supplement should be below about 45mg. Also drink a good amount of purified water everyday. If you eat a lot of fresh fruits and vegetables, your water needs are less, but make sure you're hydrated.
Do you ever get this feeling after strenuous exercise?
