Grappling takes some getting used to.
It's great cross training from striking arts because it's such a different workout.
But the body has to get accustomed to all sorts of traumas, ranging from the the very serious (e.g. ACL destruction) to, more frequently, the tiny and annoying. (My wife, who doesn't like hearing about this stuff, can stop right here!)
Since my body has become unaccustomed, I came out of my return to grappling with at least one little bruise on my arm (I never know how or when I get these), an abrasion on my shoulder from rotating on the mat underneath a guy weighing a lot more than me, and a couple of toenail slices on the top of my right foot. They're kind of like paper cuts, only from a toenail.
Three cheers, incidentally, for BJJ Steve, who recently went to a tournament as a white belt in Brazilian Jiu-Jitsu and won all three of his matches. He even has videos of his matches on his site.
I'm still trying to figure out a new martial arts schedule that will include "core," kickboxing, grappling, and still leave lots of time for things like family and sleep after my long work day and three-hour round-trip commute. But I think I'm going to another grappling class Monday night.