I have not been able to eat like I'm supposed to?

I'm 14 years old, and for the past two weeks I have not been able to eat, I will feel really hungry, take two bites then be full. I can eat about half a meal a day, if that, but I am always really thirsty. My mom says its just a phase, and she does it a lot. My dad thinks I really need to see a doctor cause I'm feeling weak now. What should I do?