Why is my muscle tone changing during pregnancy?

Very common. The demands of pregnancy will alter body composition and muscle tone. Elevated Progesterone is one main culprit. Increase in appetite, a change in metabolism, and increase in weight are common to support the demands of your growing baby. Fat storage will also increase. Don't be discouraged, keep up a level of physical activity, and you will be amazed with the changes expected after you deliver.
Exercising? Are you exercising as much as you were prior to pregnancy or did you stop?