My girlfriend is 5'5 and was weighing 172.5 pounds. My mom was going on a diet so my girlfriend wanted to also and she was very committed to it. She got to 150 pounds after about 4-5 months.

But she couldn't get past 150, and she actually gained some weight back and she is now 155. The weird thing is she gained 5 pounds within 1-2 nights and she has been eating exactly how she has been while she was losing weight. She has been working out a little bit extra, I'm not sure if you can gain more muscle that fast or not. She is still at 155 and it's been a week since she gained it.

She eats between 1250-1400 calories a day

Any ideas why that might have happened so I can help her out? She seems like she is getting down about it. Also any suggestions of how to keep losing weight would be great to.