S.W.
My daughter is 21 mos. old and I am due the beginning of April. for the past few weeks, she has become increasingly clingy and whiny, challenging my every command or request and throwing tantrums any time something doesn't go her way. She doesn't do this for her baby-sitter, or much for daddy, just for me. I do think that she can tell she is about be displaced in a way. I think about how often I hold her and how she is free to climb into my arms any time that I am sitting and not using them, lol. When her baby brother is here, that will most likely be his place, and I am sure that there will be meltdowns then as well.
I have also been told the that 'terrible twos' can arrive early and that is where her tantrums and power-struggles are coming from. I don't know, but I do sometimes hope that it is the terrible twos and that she will have passed this stage by the time the new baby arrives. She is very advanced verbally, and can have a great conversation with me. I think that as she realizes more that she can communicate her thoughts and wishes to me, the more frustrated she becomes when she can't make me understand, or thinks that I don't understand.
I'm sure this doesn't help much, but just to let you know, you aren't the only one going through it.