I'm 24 years old I have been working out for a few years now, and lost about 20lbs. Recently I have been under the guidance of a personal trainer. Since I've been working out with her I have noticed my weight going back up, also I have noticed that I'm beginning to slim ever so slightly. Any ideas??

You will gain weight as you gain muscle mass. Working out is also going to burn calories and help you reduce your body fat percentage so you get slimmer.
Ber