When trying to build muscle you don't always need to eat food. If your body needs food it will tell you (stomach will rumble). I have the similar problem for myself (However I'm a guy). I work in a leisure centre and I am in the gym pretty much 3/4 times a week now and I love it. However, I was trying to build the muscle in my chest, arm etc. but was worried about not having the size to do so (I'm only 5"8 and just about 74kg). Speaking to my boss and some PT's they said that if your body requires food it will tell you in advance otherwise don't worry too much about your diet. I've got plans from PT's and Body Builders for working your core and legs if you ever want something new to try