This could be normal.
I’ve recently had 2 hip surgeries and getting back into bicycling was really tough (and still is). My first ride (four weeks after the second surgery) was just 12km at 120W of power and afterwards my legs were burning and I was tired and exhausted for 2 days. A year ago I could do 100km at 150W without problems and wouldn’t even record a 12km ride. I’m an otherwise healthy male and 29 years old.
This is just to illustrate what a few days and weeks of inactivity can do. I went for short (2km) walks right until surgery, I only spent 10 days on crutches, I did a lot of upper body workouts (and as much as I still could for my legs), my resting heart rate was still around 40 … and yet I lost a ton of strength and endurance.
I guess if you want to make sure you should go and see a doctor to rule out an underlying health issue (e.g. heart disease).
But once that is ruled out there is really no way around normal training. Don’t ride until total exhaustion, make sure you get enough rest and protein, make sure you get enough water and carbs during and immediately after the rides (probably not that important for <1h rides), try to keep the effort steady-ish (unless you want to do interval training). It helps to record your training so you can observe trends. The good news is that starting at a low level you should see improvements quickly.
If you struggle with training sessions you had no problem with a few days ago, try to find out why. Was the intensity really the same? What about the weather? Did you change something about the bike? How did you feel before the training? (rested? hydrated? hungry?)