You don't give much information for us to work with.
But, do you think you're fat? How long has this been going on? Did anything change in your life not long before this started? How much & often (if at all) do you exercise? How is your general health? Have any of your family or friends noticed or commented on this or any other aspect of your life/behaviour/attitudes? Is this with all foods & at all hours or only with some foods or at certain meals/hours/places? Do you think you have any other symptoms? Have you talked to a dr?
All these questions affect what you may have. Like if it's recent & all the time you might just have a stomach virus. If you're not eating because you're upset about feeling like you're going to throw up it could just be stress, so if you relax it will stop (which is easy to say but hard to actually do). If you've changed the type of food you eat then it may be some sort of allergic reaction. If you think you're fat but weigh less than is average for your height then it may be an eating disorder.
Now to you for some answers so maybe we can help.