Dip in sporting ability- health issue?

[Ivyside]

Worried about my son because he has always been by far the fastest in his class, always won sports day etc. All of a sudden he looks like he’s struggling with running fast, getting tired quickly- my only concern is do all kids go through similar as they’re growing or could it be a health issue? I’m not bothered about him winning races I’m worried he’s got a deficiency or some other health problem. It seems like a really trivial thing to ask a doctor I just can’t shake the feeling of concern. He’s a super sporty child so it’s an issue I keep being confronted with as he is always doing sports.

How long has this been going on? Is it just that he seems slower than he used to, or are there more concrete things that you can point to (can't run the same distance without dropping to a walk, falls asleep/just wants to lie on the sofa after practice, that sort of thing)? Or perhaps that his running times have increased, if he's of an age and inclination to be timing that sort of thing.

If he's getting tired doing his usual activities, my first thought would be a virus coming on (or not quite recovered from one, if recently ill) or possibly a growth spurt, but if it lasts, then I'd want it checked out. Any other indicators that something's not quite right? Eating and drinking well, sleeping the usual amount, mood/behaviour fairly normal, no obvious weight gain/loss, pallor, anything like that?