When I was younger, my cousin and I would spend every weekend at our grandparents house. From about the time we were 5 years old, we knew that there were spirits in their home. We've had many experiences in that house which I will have to post later; since that time, we have both been very sensitive to spirits. We've each seen and heard things at our own homes and we can both enter a building and immediately sense whether there are spirits remaining there.
I've had several experiences where I have had to actually leave a building because of the "feelings" I would get almost as soon as I walked in. One instance was when I was a teenager and my parents were looking at houses. They found this very cute little farm house that we all loved (at least from the outside). We went inside and while our parents were downstairs, my sister and I went upstairs to check out what would have been our bedrooms. I was up there for about 10 minutes and it felt like something was pressing against my chest... Almost like there was an increase in the air pressure. I could hardly breathe. I ended up leaving my sister up there because it freaked me out so badly.
Another time this happened to me was when I was in a small town in Virginia with my mom, grandmother, and some friends. We found a cute little restaurant that was in the lower level of a 3-story home built in the 1700's. The second and third floors were used for receptions; the building was full of antique furniture and we were told that we could look around the reception rooms after we finished lunch. I felt weird all through lunch and it only got worse when we went upstairs. It was almost like there were several people trying to "speak" to me at once and I got the smothery feeling again.
All of us had kind of separated as we looked around, so I found my mom and told her I was going back outside. She asked me what was wrong and said that I was extremely pale. I ended up having to go sit in the car because I was so weak that I was afraid I was going to pass out... It was like I had been "attacked" spiritually, if that makes sense.
Almost as soon as I got outside and away from the house, it was as if a huge weight lifted off me and I could breathe again. It took me a while to get back to normal, mostly because it freaked me out so much!