What is wrong with my dog at the moment?

Your dog may still smell the smells of a ***** in heat which instinct makes him want to find her to mate. You may just have to wait for the smells to dissipate or do some odor cover cleaning. Though you may not smell anything your dog has the ability to. If his howling and crying continues I would make an appointment with the vet, as he may have something you can't tell wrong with him and the visit from the other dog may be just a coincidence. Better to be safe then sorry.