I started club crew my first semester at college and I noticed that during the most strenuous exercize, I couldn't breathe through my nose for anything more than a few minutes and I'd feel completely out of breath when I used to be able to run 5ish miles and want to do more.
I ended up going to the doctor for an ankle injury and just asked him to do a checkup, I was told I had polyps in my nose (not that uncommon). Upon seeing an ENT back home, I learnt that I had a deviated septum that was partially causing the problem and I have to get surgery to fix it on a later date.
Not saying that you have the same problem that I do, but remember something that you'd otherwise just let go as nothing could infact be something.