Why does opening my heart in Metta cause physical chest tension and anxiety?

My anxiety is immediately triggered when the guide tells me to open my heart or experience warmth there as I experience an immediate physical tension and tightness in my chest. This body resistance is as though an emotional block to guard me against vulnerability. What can I do to physically relax this part, without causing additional psychological agony? My name is Pooja, from Chennai. DOB: 19th August 1991. Time: 7:25 PM.