You should purchase "The seven minute rotator cuff solution" a good book, only about $15 and in an easy to read manner, reviews shoulder anatomy, goes through exercises that are most likely to cause impingement, and how to rehab injuries. Also, demonstates exercises. Pick it up.
Also, until you find out the cause, I would stay away from behind the neck presses, any benching lowering the bar high on the collarbone, and upright rows. While doing side lateral raises keep your palm level with the floor (don't raise your palm with the pinky side higher than the thumb). These exercises have the highest likelihood of causing impingements, although some people will have no problem at all with them. Other people have an acromion process that makes impingement more likely to occur.