Male sex drive/response is a continuum that changes from puberty all the way up to old age.  It only seems to be constant for the first 20 years or so, because in most men it's strong enough to overcome all the things on the list contained in LG's post.

But around your age, the things on that list start to make their presence felt.  I had an exceptionally high sex drive, so it hit me a few years later (around 45), but suddenly, it seemed like everything was affecting my performance and drive, whereas in the past, it seemed like nothing did.

Even things like sitting in a hot tub, OTC cold remedies, drinking alcohol at dinner, or riding my motorcycle, now had implications.  I can't speak to any medical aspect of this situation, but don't underestimate the effect that the items on that list can have on an otherwise normal and healthy middle-aged man.

Of course, you don't tell us anything about yourself, so there's no way to say more than that.  But change is the only constant regarding the male sex drive.  If you're like many middle-aged men, your awareness of what your body is telling you will increase from this point forward.  This is something women know all too well, but men usually don't think much about until middle-age.

As a humorous aside, I used to put a soft pink lightbulb in my bedroom lamp when I had a romantic evening planned, in order to make it more flattering for the lady I was hopping to see naked in my bed.  Then one day I realized it was I, not the women, that needed the flattering light to look good.  Things change and an important part of aging is learning how to deal with it gracefully.