Mild pain in chest area. Left side. Are.

Hello. I'm 41. I have been having a mild chest pain with mild back pain on and off. But in the last two weeks, I become breathless even with walking up the stairs or exercising. I have supraventricular tachycardia and I take a mild dose 1.25mg if beta blocker every day. Initially I thought it was probably my heart, but now I feel it may be my lungs. When I walk a km for exercise at a moderate pace my pulse goes upto 140. Comes down after 2 or 3 min once I stop. When I climb slightly steep stairs or goes to 112 or so. I'm not tired but I feel breathless. It's never happened before. I had costochondritis 2 yrs back. It does feel similar to that again. I felt slight rib pain but not anymore. This is mostly on the left side. My resting pulse has always been around 90 though after taking betablockers. I took blood tests recently and everything is normal. I have ibs and am on normaxin rt. My tummy is also fine though. Could you let me know what the possibilities are. thank you.