Just wondering, do you get any hands on treatment? In case the stretches are irritating your problem It seems to me not a good idea; Irritation of your problem may maintain your problem. Therefor ask your physiotherapist.
Furthermore; looking down is causing irritation seems to me more of a neck problem than Costochondritis. But having said this you have not given enough information for me to justify that statement.
Where pain? which movements pain? What sort of pain. Does coughing sneezing cause pain? and so on.