A.K.
If you don't want it to be a highschool episode, then you need to be honest. Tell her you are really glad that you are becoming friends and that you are sorry for prying into the movie issue. Tell her how you wanted to join in on the movie.
It's hard to start new friendships as women and mothers. There are different lines and different ways of showing you care and are interested in becoming friends than in highschool, when it was more widely viewed as "normal" for females to go to the bathroom together to gossip.
Also consider if this is making you feel so lousy, it may not be worth it.
I still think that being honest, while it is sometimes the toughest, is the best way. If she can't deal with it, or doesn't want to, then I think it is not a friendship meant to be. Hope it helps some.
A.