When I was in labor with my kids, I walked around the block, stood in the shower, bounced on a birth ball, walked around thr birth center (with the middle two). The only sitting I did was in the tub at the birth center, which was great. What I didn't do was lay in bed (except for a few hours with Patrick at the hospital).
Moving in labor is great for lots of reasons. What I've noticed, both at births I've attended as a doula and at my own births, is that contractions can be harder to deal with when you're laying down. I googled "contractions hurt more when laying down" and found link after link to moms talking about how their contractions hurt more when they laid down.
There are lots of different positions that the laboring mom can assume. The best thing to do is to listen to your body and keep trying different positions throughout your labor.
It may seem sometimes like your doula is encouraging you to change positions a lot during labor, but remember that she's doing it to help you be more comfortable and help your baby descend into the pelvis (and come out!)