I think the pulled groin muscle feeling is usually due to over-relaxation or separation of the pubic bone. It hurts like heck. I had it during my last 2 pregnancies, and I can tell it's beginning with this one. Yuck.
What I've learned: Don't cross your legs, and don't sit in a seat that makes your knees go higher than your hips (like bucket seats in cars). I have a wedge shaped pillow thing that I keep in the car now. Omega 3 supplements help a little too. When it get really bad, I'll ask dh to put his hands on my hips and push them together -- it feels great.
The other thing I did with ds, that I'll be doing again soon, is firming my mattress. I (fortunately!) have a sleep number bed, and I usually keep it really soft, but once my hips start giving me trouble, I pump the firmness way up.
If it gets as bad with this one as it did with ds, I'll be paying a visit to a chiropracter.