Why is intact dog suddenly stubborn at end of walks?

I'm taking care of a sweet intact male dog who has been extremely obedient and behaving very well for the last two weeks. Yesterday he didn't eat as much as normal and refused to go back inside after walks. I even took him out an extra time to make sure it wasn't potty related. I had to carry him inside after both walks (he is accustomed to being lifted and did not protest at this). But he won't go inside on his own or when called, and he wants to keep walking and will continue marking if I take him out for longer. My first guess is that there is a female in heat nearby since the behaviour change was so sudden. Could this be the case?