I had a stomachache today

Today, I’ve got a stomachache. The reason is unknown. In the morning class, I started to have a pain. It felt so bad. After I went to toilet, I got a little better. But, in the afternoon, the feeling come back. I try to think what I have eaten this day. But everything is no problem. So strange! At night, I drank some warm water and have a rest. I wish tomorrow I will be fine.