I asked WAW if she had any plans for Wed. and she replied no. I then told her I had "plans" and asked if she would be home when my Mom dropped the boys off.(mom always has them on Wednesday) She said yes, thats fine.
I'm thinking the way your W saw this, you "owed" her one for the above. So when she said she was going out on Saturday, that would make the two of you "even". When you suggested getting a sitter because you had plans too, the reason she got mad was because SHE wanted YOU sitting at home alone with the kids while she was out partying it up. You derailed her plan to "get even". Sure I may be mind-reading a bit here, but I suspect this is why she got so angry.
As for you being selfish and self-centered, sure, maybe if you told her you were going out Saturday and dealing with the kids was her problem. But you didn't, you offered to get a sitter which on the face of it seems like a good compromise. But her filter is that you're a jerk, and everything you say and do gets run through that filter for now. All you can do is try and detach from the roller coaster and keep up your GAL.
I think one difference I might suggest in the future is if you're going out again, ask her if she wants you to get a sitter or if she's OK watching the kids. Because this is going to be a hot button for her from now on.
