Hey Punky D's Mom,
So, this is what three year olds do-- they say horrible things. I'm going to tell you what I tell my preschoolers, because it's simple and effective:
"Is what she said true?" :)
If not, please don't take it personally. Kids are kids, they say stupid mean things a lot, and sometimes (very, very often) just to be contrary. I have a child in my preschool who on some days is the very epitome of contrary and will START a conversation with "No it's not" before anything is even said!
All this to say, your daughter still loves you, and I think you need to not read too much into it. I'm sure she's already forgotten about it. The way you don't let it ruin your day is to remember that this likely isn't about anything deeper than her either playing with reality for a moment ( a lot of imaginative and pretend play develops around this time) or that she's just seeking reassurance.
Or perhaps, she was mad, and if she says this when she's mad, then you can respond with "You know, that isn't true. If you are upset, please tell me what's bothering you." and move the conversation past this little odd aside.
Now, here's the other part: you don't need to go back and talk about it or correct it, this time. Making this a conversational focus is a guarantee it will be repeated. The next time she says it, just look at her and say (not upset, no big emotions here) "Oh, no, that isn't true. You grew in my tummy and came out of my tummy.Want to see?" Then, go look at some baby pictures of the two of you together. Warm, cozy feelings will soon take hold.
For what it's worth, I have seen lots of kids go through this phase/statement, even in homes where there are no girlfriends or divorce to wrinkle and complicate these moments. It's pretty par for the course. Sending you a hug, because when we aren't prepared for these moments, man, they can pull the rug out from under us!