General

Thanks everyone for the suggestions I think i will go to see the doctor soon. Since i do year round sports, i cant be having random days where im weak and then random days when im like stronger than i normally am. Thank you basil!

Because you don't push yourself hard enough

maybe you have low glycogen.

hmm if it is an iodine issue, eating anything with salt should do it. most salt comes with iodine nowadays lol.

but primary thyroid disorders are pretty uncommon because of how prevalent iodine is in the foods we eat nowadays. generally, secondary disorders are far more common which usually has to be treated through the help of thyroid hormone pills. I wouldn't suggest taking iodine pills yet because you don't know if it's primary/secondary disorder, or even if it's a thyroid disorder at all. overloading your body with iodine could cause the thyroid to go a little crazy and start kicking out more hormones than it should. the point is, if you don't know, don't start anything yet; for now, focus on figuring out what it is that's going wrong.

So if anything, go to the doctor's and get some bloodwork done.

If this is a one time thing, i wouldn't worry about it. if it's recurring and you find that you're always dipping in and out, it could be any number of things. As @ZaraByte said earlier, it could be a thyroid issue. I'm not so sure tap water can affect the thyroid but if you're drinking the same water all the time, this shouldn't be an issue that just comes out of nowhere. But hypothyroidism is a common disorder and it results in decreased metabolism, which in turn can lower your energy efficiency/usage. But again, unless you've had some drastic change in lifestyle/dietary habits, it shouldn't be an acute condition.

Maybe it was what you ate that day, etc. If you're willing, describe as best as you can what you felt/what happened when you felt you couldn't do beyond 20 pushups. It's not likely but maybe your muscles that day weren't responding, you had a sudden and rapid decrease in calcium in your body, or you had already been doing some exercise that day and your muscles at that point were already fatigued.

If this is something that's happening all the time (hopping between sudden weakness and then feeling fine again) I'd say you should visit a doctor. Not to scare you but if it's as sudden as you described it, you might even have a heart condition. If you're an athlete, I believe most schools (and college institutions, especially) want students to take an EKG. But yeah, definitely get a physical exam if you suspect something weird is happening.

But again this is a worst case scenario. If it's nothing as serious as I think it is, it's most likely just fatigue.

Have you consistently drank enough water?

i have a hard time believin' eatin' habits haven't changed.

maybe it's the weather that's affecting your body