I've been thinking almost similar thoughts - although DD doesn't know I'm 10 weeks pregnant (DD is only 11mths).

When it comes to feeding she's an angel for her father, for me she's a terror; when I pick her up she hits me (playfully) but she doesn't when DP holds her! But I've noticed a change over the last two days when DP has been at work and she hasn't had the chance but to be with me lol!

Maybe do 'story time' with her / or light activities where you don't get too exhausted. Does DD sorta understand a baby is on its way? Could she be feeling a bit unsure of where she'll 'fit' into the family when your bubba comes along? (Sorry, I don't have a child your DD's age so I'm not sure how understanding they can be at that age )...