Sounds like stress to me. Digestion is usually one of the first things to suffer when we are stressed. Some people over eat, others can't eat. It sounds like you aren't living at home. Are you away at school? Being separated from home and family can be big stress inducer, especially for the first time. Stay hydrated and try drinking some gatorade or powerade, only dilute it 50/50 with water so it isn't so sweet. Good luck and God Bless.
It sounds like you are depressed and still homesick. You might want to see a doctor about some anti-anxiety meds. Young people in particular seem to hide their feelings or to "swallow" them but they show up somehow. - When you try to eat, make sure you think of something pleasant and try not to dwell on what it's like at home. If possible, try not to eat alone. A few sips of water just before you eat might help also.
Hope this info is helpful. Take care, because we care.
Is there any possibility of you being pregnant?
The other reason may be that you have a tummy bug. Even though you don't eat, make sure that you drink plenty of fluids. Soups, juices and water. You do need to make sure that you do not become dehydrated, so it is important to have juices and soup. Whilst you are feeling unwell avoid milk and dairy until you feel well gain.
It may be a good idea to see the doctor to make sure that you are not anaemic, have any stomach problems and a pregnancy test if that may be a possibility.
Best wishes.