What's wrong with me? How can I stop this?

So I have this problem.. I can't sleep over at a friends house.. even one of my closest friends... no mather where we are I have trouble sleeping in a room I have never slept. I just get really sick to my stomach and I feel like throwing up. One time I actually did throw up. I had to stay at my friends house for 5 days. First 2 days were horrible. The rest were perfectly normal. I want to sleep over at my friends house.. what can I do if it happens again?
What's wrong with me? How can I stop this?
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