Hello. Thank you for coming to JustAnswer with your question, and welcome.
Flomax has a listed side effect of "abnormal ejaculation", which includes decreased ejaculation, ejaculation failure, and retrograde ejaculation. In the clinical trials done, this side effect seemed to be dose related, which means a lowering of the dose and stopping the drug may reverse the situation.
However, this medication may be very necessary for your health condition being treated, so you should discuss this with your doctor before deciding to make any changes.