First post - tired after morning sex

Does this sound normal for 39? Does this sound like maybe a hormonal problem or maybe even a sleep disorder or would you blame it maybe on my age? Nothing else has changed in my life. (work habits are the same, eating/diet hasn’t changed, not overweight) Each night, I do get my 7 to 8 hours of steady sleep.
But, each morning immediately following sex (specifically after I orgasm) in the early morning I become so overwhelmed from exhaustion that I cannot get myself even out of the bed. I usually have to lay there for some time. Now, my husband and I have been together four years now. He is 29 and I just turned 39. I have never been with anyone else but him. This is the same pattern/routine we have always followed. On the weekends, we do have sex before going to sleep as well, but normally we follow this same pattern. And, I am definitely not pregnant. I noticed this change recently-- I am guessing within the last 10 weeks. One of my sisters blames it on my age and the fact that I didn’t become sexually active until I was 35. She thinks I can’t keep up with my younger husband. Hoping that isn’t the case. Thank you for listening and for your help.

First post - tired after morning sex