Can anyone give me advice on what I think is an overactive imagination disorder?

Okay so, I have been known to pass out. A lot. At first, doctors just thought it was just low iron or dehydration. I believed them at first, but now I know that this is not the case.

This is going to sound strange, but when people explain things to me, tell stories, lecture, ect. I over-think what they’re saying. When it’s just the person speaking, I get these crazy visuals in my head of my take on what they are explaining to me.

Most of the time, this happens in classes involving the human body, like Health and Biology. And they are never a good thing. I exaggerate possibilities to the point where I am basically having nightmares while I am awake.

But what is weird is that, I know in my head that these are just fantasies. It’s like my mind is sane, while my brain is not. I try to fight these spells by attempting to focus on other things and block it out. But somehow, my brain, my body, takes over and completely shuts itself down, almost like it’s trying to protect me from danger.

I’ve been told by some that this is just anxiety. Panic attacks. But I don’t have any reason to feel anxious at all. I’m not an alcoholic, addicted to drugs, and I’ve never had something traumatic happen in my lifetime. I am a very relaxed person, I don’t feel stressed very often. And if I do feel stressed, it’s over something minor, like schoolwork. And I’m not paranoid about anything.

It’s like my brain/body is anxious, but my mind/personality is not.

Can anyone explain to me what this is, or give me some advice?