Hmm. I would need a better description, but from what you have said, i think you are dealing with a shadow person. Shadow beings take the shape of people and have been known to basically suck the life out of their victims and cause immense fear in them. This can be life threatening, and it is very serious. Of course, this is only my opinion, and i could be wrong. I suggest you protect yourself, and do a banishing ritual and a house cleansing.