Should I stay or should I go?

We all attend parties whether by formal or informal invitation as long as the host personally extends an invitation, which is usually several weeks or days ahead of the party. But, what happens if the invitation was made at the last minute? This is my dilemma. A phone message was left by one of my former officemates inviting me to her husband’s birthday – supposedly a surprise event for him that she’s been planning for a while. The main issue here is why are you inviting me on the Thursday before the party that is a Saturday dinner at a restaurant. I’m thinking that since it is at a fancy place that she would have sent out invitations, right? I feel like an understudy right now considering that perhaps not enough guests sent in their RSVPs and they are now short on guests. Hmm. I know I may sound like a prima dona, but I really feel the consolation prize at a raffle. Yes, it was nice of her to invite me, but she should have contacted me at least sooner, since calling me at the last minute seems a bit desperate.