What gives?

I went to the gym today and started out on the treadmill. I actually felt pretty great running. I was going really slow but that honestly doesn't bother me because I felt like I was getting a workout in. But again, right around 3/4 mile I started to get a lot of itching on my upper legs and butt. So freaking embarrassing since I know it's pretty caused from my inactivity during the first trimester. I hopped on the cross-trainer after about 2 miles and it didn't happen on there. So maybe if I keep this up my fitness level will get back up there and the itchiness will go away. If not, I will just keep it up on the 'ol cross-trainer. I have a feeling that within a couple of months I'll be on that anyway since I'm already starting to have some issues with my pubic symphysis. I'm trying to do some core work but it's already getting difficult.

Speaking of core exercises, does anyone out there have any good core exercises to use while pregnant? I am doing the plank and it is getting tough already. :-) That's how weak I am! But I can still do crunches on the ball so I guess that will have to work for now.