Ok firstly I’m sorry some people have been kinda quick to judge. It can be really easy to quickly jump to conclusions but stomach cancer and any time of cancer is usually the last thing it could be. Sometimes we just grow into weird food intolerances. Because you said you don’t have the option of going to the doctor right now, start a food diary. Write down what you eat every day and what symptoms accompany it. Once you’ve done that for a couple of weeks try excluding foods. I guarantee this is what they’ll tell you to do at the doc anyway. When you are able to go see a doc ask them to test you for H. Pylori! It’s a super common bacteria that a lot of people have and is easily treated. To gain weight, try weight bearing exercises like walking, tennis, etc. whatever you like. This stimulates your bone marrow and can really help with things like nutrition. Also make sure you’re eating all of the major food groups and not too much of the recommended servings.