On becoming a big sister...

We just had a second baby a little over a month ago, our first is 22 months old. At first, she did really well with having a baby brother, and still does mostly. She doesn't quite understand how much bigger/stronger she is than him, so we have to have that talk with her occasionally (no jumping on little brother, don't hit (she thinks she's patting like burping but it's too hard), etc.

Just recently though, her mood has shifted kind of dramatically. She's throwing more tantrums, become slightly despondent at daycare pickup (she used to run to me with big smiles and is now more hesitant to leave), and overall more withholding. I recognize this could also have a lot to do with her age, but has anyone else experienced this? What's your take? How long can we expect this to last? Anything we can do to help ease this transition?