I had this on my right side, it felt like my intercostal muscles in between my ribs. My doctor told me it was Osteochondritis. There isn't anything they can do except prescribe painkillers which I couldn't take anyway. It turned out I had Polyhydramnios (excess amniotic fluid) which is why my ribs were stretched and hurting so much. It was just on the right side though, it hurt more when sitting, driving only relief was bein lay down.