If your riding/training experience has changed for no clear reason and your fitness has dropped more than you would expect (you didn’t say how long this has been going on) GO TO THE DOCTOR.
My wife is training for an Ironman. For about 12 months her performance drifted down for no good reason. “Over training”, “smoke from the wild fires”, “stress from work”, “stress from the pandemic”, “a bad lunch”. All sorts of ways to explain why today wasn’t a great work out. We never looked back at the data to see just how badly her performance had suffered.
Then she had some tightness in her chest. Nothing “serious” but after 3 days she decided to visit urgent care. Nothing seemed wrong and they took some blood. One test (D-Dimer for you playing along at home) was marked stat. This was a Friday afternoon. We joked that as long as the doctor didn’t call on Saturday it was no big deal. 9:05 Saturday morning the phone rings. 30 minutes later we are standing in the ER.
My active, heathy, fit wife had multiple massive pulmonary embolie. That’s blood clots in the lungs. Fortunately, she has an incredible aerobic base. She was and is fine. She spent one night in the hospital (ICU according to the bill) while they started her on blood thinners. The nurses came it to marvel that she wasn’t on oxygen. Her hematologist jokes that he doesn’t see very many healthy people.
She’ll be fine after she completes the course of blood thinners. OTOH, if she had ignored it she could have died. We sat and read her medical file while I had an image of the pulmonary system on screen. She would read out where she had a PE and I would point to the screen. It was a long list.
Don’t ignore potential warning signs. It might be nothing. It might not be nothing.