Do you ever feel sick but can't explain what's wrong?

I get this feeling sometime where I just feel sick and I only want to sleep. I lay in bed crying because I feel horrible, but I can't say that anything specifically hurts, and I'm not throwing up. I will sleep hours more than I normally do, or need. Has anyone experienced this or know why it happens? I just feel so sad and bleh and tired and sick all day but I don't know what's wrong. How can you go to the doctor with no symptoms for them to start with? If you have ever experienced this or know why this happens please comment!
Do you ever feel sick but can't explain what's wrong?
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