When you say you "eat something", do you mean a 'something' with sugar or flour or starch or potatoes in it? If so, then your body will retain water. That's what carbs do: they make each cell of your body hold onto some water and there will also be water in the intestines. That's why many people lose a lot in the Induction period or first weeks when they cut out those "bad" carbs.
I see that you are at 125 pounds, aiming for 120 pounds. That is awfully and amazingly close to your goal. You should not expect to lose those five pounds quickly and maybe that is too low a goal anyway and your body is trying to tell you that. The rule of thumb for height/weight relation is: 100 pounds for the first 5 feet of height and 5 pounds for every inch over that. For 5'7", the total would be 100 + (7 x 5 or 35) = 135 pounds as ideal. And you are under that now. I know a 23 year old who is 5'9" and weighs 170 and she is a size 12 and beautifully slim.
You say that you 'lose 15 pounds in 2 weeks". I hope that is not recently because that means you must be eating in a way to gain about 10 pounds quickly, then losing it again. That would be called "yo-yo dieting' and it isn't good for any body - even one as close to goal as you are.
A food sensitivity would take many months of avoiding the food to reduce the sensitivity, and it might not work anyway. It is fairly easy to check out your own sensitivities by going to a very simple, plain diet for a few days, then gradually reintroducing a SMALL amount of one food you think might be a bother to you.
The most common foods to cause problems include: milk, wheat, eggs, corn, yeast and a few others I can't remember right now. So you could try eliminating those and then try a bit of one of them after a few days.
I hope this helps. I am sure other people will drop in with more advice.