Most of the time, no noticeable permanent damage, if any, will occur. That is unless you have prior medical issues.
In general, losing consciousness while pressure is still applied is what will cause permanent damage. So if you passed out for say, 1 minute, it could possibly cause some damage. But applying pressure for 10 seconds to get light headed or "high" (some people enjoy the feeling. It's called the choking game in that case) generally doesn't cause issues. Blood clots could be possible like @Suez stated. I truly don't think it's likely. If you ever were to practice karate or taekwondo(I did taekwondo for years), they will have you practice the choke hold. My point is that it very rarely causes permanent damage. It's usually temporary.
I've felt weird in my head for a couple of days once when practicing partial. I will sometimes also get a headache. That is normal. Light headedness is also normal.
FYI, if the belt is secured to the door, it is very risky(assuming you don't want to die yet). Not only could you slip out of consciousness faster than expected(especially if you position it "just right" one time then you could lose consciousness faster, potentially, compared to other times) , you could also trigger the vagus nerve response(there are other names for it). Once I got mad and squeezed my carotids with my fingers as I had done many times before, but I passed out. My vision went back to normal when I released my fingers, but then after 10 seconds, my heart beat weird, like it skipped a beat, and I passed out. If you do this in the noose, you'd probably be dead. Just be wary of this.
I wish you the best. We're here any time you need to talk.