I was selectively not invited to a coworkers retirement party.?

I know that she did not pick the guest list. It is a company sponsored event. I have known the woman since I was born, and have been working with her for 6 years. There are other people invited that have not known her nearly as long and/or don't even work for the company anymore. To add insult to injury, I am being asked to chip in for a gift from our department... "Whatever you want to give" I know I should suck it up and help pay, but I'm still a little upset I'm not invited. What should I do?