So the other day I bought a 99$ bike from Walmart. Allegedly a mountain bike.

I barely rode it for 1.3 to 1.5 miles and I basically collapsed on the side walk. It felt like I ran for 1.5 miles. I had to lay down on the side walk for 10 minutes before I could get up. I don't know if it is because:

1. I was not using the correct gear? I was riding it on 3rd gear. The ground was totally flat.
2. It was too hot? It was around 40c in Austin, Texas.
3. I am out of shape. I am lean but I don't exercise a lot.
4. I don't know if the bike is garbage.

I needed a commuter bike to get to work which is 4.2 miles away from home. It just didn't seem practical so I returned it.

I don't want to buy an e-bike because I would have to carry it up 2 flights of stairs.

What do you guys think went wrong? If it is something fixable, I might consider getting the bike again.