Symptoms of a pinched nerve?

I have this tingling/ticklish feeling that forms around my left nipple, my left underarm, and it radiates to my neck. At first I thought it was acid reflux but now I'm not too certain. I think it might be a pinched nerve because the funny feeling is relieved when I sit slouching, as oppose to sitting up straight. And when I'm laying down, I feel relieve when I sleep with high elevation of pillows. Where would the pinched nerve be if this is the case? Thanks yo.