No it's not constipation, not the way we usually think of it, anyway. Her cancer is in her colon, so anything passing through has to get past the mass (es).
Her poops are, for the most part normal looking, once she gets them out. A couple of her litter box trips might result in small mushy pieces, instead of formed, but that is happening less often, almost all are formed, even the tiny ones.
But sometimes she goes down and stands for a long time and produces nothing. She doesn't strain when she does that she just..stands there as described and eventually leaves the box without going into the poop crouch and push. That of course is the cancer making her feel like she has to go, even when there isn't anything to go.
In her notes the IM vet did mention that she may need lactulose at some point. I hope not, I loathe that stuff and I know Jennie would, too.