Did I do something wrong?

Well, you could have suggested she come with you to bring your son to school, given how your ex did the same with her current bf. She might feel a bit left out.

My suggestion? Try to talk about it, ask her directly what's bothering her.

Unfortunately, we live 40 minutes apart and she has two kids of her own to take to school at the same time.

I don't think you did anything wrong. I am in a similar situation. I have a daughter who has a mother, and I have a girlfriend.

My girlfriend understands that I have co-parenting relationship with the mother of my daughter. My daughter is important to me. Keeping a cooperative relationship with the mother of my daughter is important to me. I go with her mother to my daughter's performances, meetings with teachers, presentations, girl scout activities. It is all part of being a good father... looking out for what is best for your child.

Your girlfriend is being ridiculous. If she can't accept that you are a father, then she shouldn't be dating someone with children.

You did the right thing for your son and that's the important thing.

Your son started preschool and he deserves to know that his parents can celebrate his important moments together.

Does this off/on woman have an issue with you being a good parent to your son? if you think the off/on relationship is going to be more than occasional sex/whatnot, I think you need to have a serious sit-down with her and talk about the importance of your role as a father. Part of that role means being able to share good moments with your son's mother - to help him have a solid base - and some terrific memories to look back on.

that might be a good thing eh

Drop her.

She does not understand the importance of cooperative parenting.