I feel that there's a lot of information missing here. Why can't you talk to her and ask her what's up? You don't need to call her out or start a fight about it lol... just simply say "Hey I notice whenever we have plans you cancel ... is something going on?".

Do you think that maybe she doesn't have the money and is only realizing it after getting paid or an unexpected bill? Are all your plans revolved around going out (or the wedding for that matter)... why can't you do something at one of your places? Some people aren't comfortable just coming right out saying "yeah I'm in a rough spot"... or when she says she doesn't have the money it's because she has other priorities or other things that come first (bills, family commitment she has to spend money on, larger commitment with friend etc). I've done it before but I'm also comfortable telling them why.

It doesn't explain the night she went out on snap chat unless someone offered to cover or she didn't actually buy anything. But if you talked to her I'm sure you could find out... maybe there's a reasonable explanation OR maybe she is being a jerk.