Dimmesdale did indeed have a scarlet letter on his chest.  And, it was he himself that put it there.  He gave himself a tattoo of sorts.  The guilt of not bearing Hester's burden of shame with her, the guilt of his sin, the guilt of his cowardice in not confessing his sin to the world, drives him to injure himself physically.  One of the ways he "makes amends" for what he has done is to carve a letter A into his flesh, on his chest, so that it scars over, forming his own personal mark of shame.  This sort of twisted "repentance" is his way of punishing himself, of bearing pain as a way to pay for his cowardice and sin.  That is why, in the chapter you referred to above, Chillingworth is certain that he had found the man Hester cheated with.  As Dimmesdale was sleeping, he "shuddered" and slightly stirred," which must have shifted his position enough to have his blanket and coverings slip away just enough to reveal the A.  Chillingworth saw it, and, demented man that he is, experienced "ghastly rapture" at the discovery.  So, yes, Dimmesdale has an A on his chest, a self-inflicted wound that is evidence of his guilty conscience, and Chillingworth discovers it, confirming his suspicions that Dimmesdale is the one Hester cheated with.  I hope that helps a bit; good luck!