Why could i have pain under my breastbone in my ribs when i lay down? hurts bad if i get up it goes away? i have polymyalgia could this be a sign of a flare..thank you?

There are various causes of pain in that region, but your discomfort appears to be positional and possibly muscular. Any recent strain likely? PMR doesn't usually affect abdominal muscles. If this pain persists,&especially if there are other symptoms like tenderness,digestive disturbances,reflux,etc you should be examined & advised by your physician.