I agree with everyone above, that even if you don't end up changing tomorrow's movie plans (I think you should), in future please say to her: "Oh, OK, if you don't want to come, we'll catch you next time." Or suggest she organize a separate outing to the thing she wants to do.
In terms of expanding your circle, can you join an organized activity in something that interests you? Sports (or something else) at the local community centre, a hiking club or walking group, or cycling club. Doesn't have to be a physical activity; could be a book club, photography club...anything really. But not a class; classes are great, but I find they don't always lend themselves to making friends outside of the class time. Everyone just goes to the class and packs up and leaves after without really getting to know each other (not always the case, but more likely).
The key is to join an organization where everyone's actively doing something together, and you see the same people week after week (with new people coming in all the time).
I say this because my social circle expanded in a phenomenal way after I joined a hiking club. We'd go for hikes, and naturally talk, and then (being super hungry) all go for dinner after. It wasn''t always the same people every week but as time went by, you did tend to see the same people again and again. Next we were organizing our own outings outside the club, and then movies, and dinners at each others' homes, games nights, and other activities. And then those people would bring friends...you get the picture. This was years ago and I've come to the realization that most of my social circle origiinated with that group, even though most people aren't involved in it anymore.
You could try Meetup.com in your area as there are groups for every possible thing you could be interested in (depends where you live, I guess, but if you are in or near a major urban centre I would imagine you could find something.
Or, do what one of my friends did (he was more of an aquaintance, really) and send out an email to a number of people you'd like to know better, and say, "I'm organizing a dinner at Local Restaurant which I've been dying to try. Bring a friend if you wish. The idea is to try a new restaurant while meeting new people." I had only met this guy once and I attended his dinner. I am now friends with one of the women that came as we discovered a common interest and "friended" each other on Facebook afterwards. People are remarkably receptive to being invited somewhere. Perhaps some people would be shy about showing up to a dinner where they barely knew people, so it could be bowling instead, so that everyone's active instead of trying awkwardly to make conversation.