One thing to consider is that it sounds like L does not have a very stable personality in the best of times, and now she (and you) has post-pregnancy hormones to add to mix. Some people react more strongly to hormonal changes, and she may be one of them. This may make her even less stable. So her behavior is probably less about you and more about her ability to cope in general.
That said, I empathize with your dilemma. While I do think it is beneficial to give others the benefit of the doubt and be accomodating as possible, I had to learn the hard way, that rare individuals will allow me to do all the accomodating and ask for more, while he or she gives nothing and are totally offended at the thought that I might expect him or her to actually compromise and contribute. L sounds much like the person who taught me this lesson. Perhaps, she is not; I hope she is not.