SS, here’s the thing.
I know you’re a young dude of 20, but even at that age there is a slight leveloff of hormones for most guys. The insanity of erections I got in HS had started a slight tail by about 20 or so. Not that I had any erection issues. But it does not stay in that ‘hyper’ mode for long. Be glad, dude, because you will build staying power.
The reality is that our bodies fluctuate from day to day in their responsiveness, especially to sexual stimulation. It’s actually quite normal and I do not think it is PE related whatsoever.
The male erection capacity is actually a very complex process, easily disturbed by many factors. Even if you feel you as a young lad have NOS running through your cavernosa.
I say, sit back and stop worrying for now. Don’t judge day to day as you’ll only freak yourself out if it doesn’t seem as stellar as before.