Hi Kelly
I hope you are ok and I really hope all is well with your baby. I know people here try to reassure eachother by saying things like "chances are your baby is fine" and things like that but I seriously think you should go to your doctors as soon as possible and get this checked out. Please don't panic, I dont mean to alarm you but my bf is a builder and he has knowledge of lead in paintwork and he thinks you should seek medical advice asap. Also if you still have any of the paint chips left you could contact an organisation and get the paint tested for lead traces.
I agree with Becky with regards to the method of sanding so at least you would have only breathed in a small amount of the dust if you were sanding by hand, not the amounts you could have inhaled if you were using an industrial sander etc.
I don't think beating yourself up over it will help and you wasn't being selfish, its just something that wouldn't occur to the majority of people, I expect, and you are no different. The only difference with you is that you have found out the risks now and are in a position to get medical advice and hopefully remedy any possible damage this could have caused to your health and that of your babies.
I wish you the best of luck
lou x