It's normal after a long relationship to fall back into each other's arms. When you break up and you are confronted with being alone and looking for a couple, it's natural to relapse into the past relationship, because well... who knows you better in bed than the person you were married to?

The strangeness of it all and the contradictory message she is sending is maybe a sign that she is as confused as you about this, but she's coming from a defensive position (most likely because she left you), while you are not, which explains why she might be cold before and after.

Would you like to get back together? Have you told her how this uncertainty is making you suffer? You should think about what is best for you, so that you can be a good and stable father figure for your daughter.