Help! My dog is having trouble pooping!

Took my 3 1/2 year old standard poodle to the park today where she usually poops after running after the ball for a while. Today she sqautted several times and seemed to be straining - but nothing happened. Since then, we've been outside gardening for several hours but she didn't try again.
Talked to my neighbor about this and she said she'd noticed that Bella was having the same problem last night. (Squatting, straining, no result.) Bella is my first dog. I've had her for 2 years and this has never happened before. Otherwise, she seems fine, not in any pain or discomfort, even when I palpate her tummy. She ate a little bit of dinner, but then, she's never been a chow-hound.

Should I be as worried as I am? Is there something I can do?

Thanks so much.