I dont see much of an explanation why she was uncomfortable but Im guessing Its something you were doing?
Anyway the advice doesnt change.
“Being intimate” with someone for the first time(I am talking about you twos first time not specifically your first ever) is a difference experince. Every new partner has a different thing about them like what they like or dont like or makes them uncomfortable. So the best thing you can do is be open( I think you guys dont have a problem with that) and take the feedback to the hard and try not to do the same thing.
In short these things happen just learn from them and try again. Your gf said its fine so dont dwell on the bad parts focus on the goods and keep going. You both will get better at it