Our Boston is due June 12th and she really shows! It's not so bulging that we think she will pop but you know she is having babies. Her nipples have grown very much in the past week too.
Some momma's that are carrying only 3 or 4 babies may not show very much at first. Give it a couple of more weeks and she should get bigger in her stomach area. You could measure with a tape measure and then you may see more growth than you notice with just looking at her. Also, keep weighing her every week. Her weight should be increasing right now too.
Be sure to see the vet if you haven't already. He needs to take a stool sample and he can tell you for sure if she is expecting.