work dilemma... am I being selfish?

Ah
I work retail management and my official last day was supposed to be November 20th. I gave a 5 week notice and I would've been 36 weeks. They didn't find anyone to replace me and I was feeling ok so I said I'd help out for a few weeks extra. Last week I'd get terrible contractions at work and spent most of the time sitting in the back room to get through them. Saturday I had my bloody show and today I've moved from 1 to 2cm's dilated and 50% effaced. Contractions have started back up again so I told my boss I'm done working because I really don't want labor to start at work. Her son is leaving for the marines on Sunday and I was supposed to work that shift for her but I honestly think it's a bad idea for me to keep working. I still have things to do at home before baby is born. I feel really bad about not going to work but I feel they should've had a back up plan considering I'll be 39 weeks next week. And my last day was technically the 20th. Am I being selfish? I hate telling people no but I honestly would be miserable at work and I've already helped out past my end date