Randomly not napping in crib

My son had been napping perfectly since 6 months (he is almost 13 months). Normally we change his diaper, get him a bottle and lay him down in his nursery with the blinds closed and his sound machine. Today the second he saw we were going into his room he screamed and threw his head back. He refused his bottle. I tried to do the ferber method because its how we sleep trained in the first place but it wasnt working so i ended up rocking him in my rocking chair to sleep. He isnt sick or acting strange at all otherwise. Anyone know whats happening? Edit: he got sick a few hours after this post. Fever, stuffy nose etc so it was definitely that