Best friend not acknowledging my birthday

My sister in law invited me to go with her to the spa for her birthday (along with 2 other women). I declined and didn’t offer another possible date because we do not have the money. It probably looks like money isn’t an issue because when it was MY birthday, my DH gave me a spa day. But we saved for that. We can’t afford multiple spa days, especially if we’ve just decided to tighten the belt a little (as we do once or twice a year). You don’t know what’s going on with her even if you think you do. Let it go.