"thought it was a muscle spasm"

Sharp Pain with Every Breath: Intercostal Neuralgia, a Warning from Your Flank

Are you losing sleep at night because of a sharp, knife-like stabbing pain every time you take a breath? Feeling that stinging ache travel from your flank up to your back, you might have worried it was a serious illness. You are not alone in experiencing this. Let us listen closely together to this small signal your flank is sending.