It is definitely not normal to feel nauseated when thinking about food or to lose weight without trying. Therefore, I highly recommend seeing your doctor to help you figure out what is going on.
One possibility is that you are pregnant. This is a common cause of unexplained nausea and loss of appetite in young women and, if there is any possibility of this at all, it is worth checking it out with a pregnancy kit available in any local pharmacy.
There are multiple other possibilities as well. For example, an inflammation of the stomach or a stomach ulcer can cause nausea or pain with food. The chances of you have a medical problem in your gastrointestinal system goes up if you are experiencing constipation, diarrhea, vomiting, or abdominal pain along with your loss of appetite.
Another possibility would be a mood problem, especially if you have been experiencing sadness or loss of motivation, trouble sleeping, loss of interest in work, hobbies, or your sex life, etc. These might all be symptoms of depression, which can also cause loss of interest in food and weight loss.
Your should talk with your primary care doctor as soon as possible to help you figure out what is going on.