Is it just my metabolism?
Partly age, mostly what you stopped doing. Fewer steps, less lifting and worse sleep move the needle more than a birthday does.
What about testosterone?
Low testosterone can contribute to fatigue and body changes. It is worth testing if you are tired despite sleeping seven or eight hours. See what was discussed on low testosterone and ask your doctor.
What is the fix?
The same four habits. Lift three times a week, walk toward 7,000 steps, eat 0.8 to 1.2 grams of protein per pound of goal weight, and protect your sleep. The full plan is in How to lose a dad bod.