She should be fine. I have fed too big of a meal once due to asking for one size and receiving another. I would just make sure she had normal heat like has already been mentioned and just leave her alone for a few days.

As far as the other mouse being pregnant for 3 months. That is not possible. Mice are pregnant for about 3 weeks, give or take a few days. If she is continuing to gain weight and she looks pregnant, she may be obese or be sick.