I couldnt get my penis in full erect during sex?!

It's performance pressure. Down in the basement you weren't thinking about it, so it was happily getting hard as possible. Upstairs it was showtime, so you had a little stage fright. The fact that it wasn't responding as fast as you wanted caused more anxiety, which made your hard on even more elusive. It's a vicious cycle that's very hard to get out of. Your full erection will return when you don't care if it does.

Lots of people lose their erections after putting a condom on, and especially after running up the stairs.

It kind of takes the "heat out of the moment", and removes the blood pressure from the penis.

The trick is to open the condom FIRST, and then get hot and heavy. And not to go running up the stairs :D

At the time u were in the basement the moment was better than it was upstairs.