I wonder if yoiu might have retrograde ejaculation? That's when the semen is ejcted backwards into the bladder rather than out thru the penis. It's not necessarily harmful, but doesn't feel the same & is usually not as pleasurable as a regular ejaculation. You said you went for a checkup - was that with your regular Dr? If so, maybe see a Urologist. Maybe some further tests can get to the bottom of this...