Ok man, you are wise to want to learn from this situation. First of all, you were sitting on a couch? What couch? Your couch, her couch, a couch at a party? If you were at a social event, or even a small gathering, very very few girls will allow you to do that with others around. They are out there for sure, but thats for another thread.
We will assume you were at a private location. Girls will stop you from escalating for various reasons, but the number one and by far most common is thier instinctive ASD, or anti-slut device. This is a little red flag that shoots up when they are not comfortable with you yet. Don't take it personal, happened to me last week. This reason on its own is a bad reason to call off all future 'rapport' with the girl. Shes just not warmed up enough towards you. Don't just quit dude!
What you should have done was say "Ok, thats totally cool" then gone back to making out with her, and to touching her (caressing her

) where she was allowing you to. She liked making out with you right? She was on top of you right? Don't get huffy-puffy when she draws a line, this shit is an art of finesse, not a grope fest. A lot of guys will do whats called a freeze-out, which holds a certain amount of merit, but I personally find it childish. Maybe someone else could explain it, or you can search it out yourself. It can be hit and miss, at least it was when I played around with it.
Myself, I go the 'two steps forward, one step back' route. So shes not hot yet. The idea is to get her to the point where she is begging you to peel those panties off. Learn about erogenous zones. Hit 'em up like 2pac! Learn to be sensual. Don't suck at kissing. Nape of neck, earlobes, inner elbows amongst other places. Get her wet! Remember, chicks love sex more than we do. Its true. Also, soft spoken 'no's or 'uhuh's = 'not just yet...' Hard spoken 'no's mean 'step the fuck back.' (thats important to note) Hope this helps.
Edit... I know I mentioned ASD above, which plays a serious role, but that applies more towards social settings. What I should have said, and more specifically to the point I believe is whats called "token resistance.' Thats when the chick resists cus she is programmed to not be easy. Silly girls!