I had bad burning pains behind my arm also but they settled quickly and within a couple of weeks I didn’t notice them anymore. The soreness/ tightness under my arm with movement lasted longer but I am 7 months down and don’t have any discomfort at all now and I haven’t had any for a long time. Don’t despair yet - you might be surprised how quickly you heal and the pain settles down. The first weeks are hard because it’s all unknown and it’s an unexpected pain but for me anyway it all came good. Be kind to yourself and give it some time.