Suddenly crying when I lay him down to sleep and don't know why!!!!

My 6 month old will not stop crying... only when lying down. He's never like this at all. He's fine when being held. We've been trying to put him to sleep for 4 hours now............. We've tried EVERYTHING. I even gave him Tylenol. I don't see any teeth, but think I MAY feel a protrusion?? It's hard to tell bc he keeps moving and refuses a pacifier, cold teething toy, cold wet washcloth, etc. ***** help *****