Dogs aren't stubborn - that's us anthropomorphising them because it's often easier to say "He's being stubborn" than it is to find the reason behind the behaviour. Most reluctant behaviour can be explained by the following:
Fear - maybe something has happened to them at that point on a walk before, and they're scared of it happening again. Maybe they can smell or sense something you can't, and it's upsetting them.
Pain - maybe they dislike the surface you're walking on, or their harness/collar is hurting them, or they've injured themselves in that area before and they're worried about doing it again. Maybe they're feeling unwell or tired and are just wanting to go home instead.
Lack of motivation - some breeds need motivating differently to others. Maybe they don't like the weather, maybe they know if they turn right (instead of left) they'll end up at the park, or at the pond, or somewhere they know they'll see their friends, or where they'll be able to go off-lead etc. Similarly, some dogs "refuse" to go back to the car after walks (or refuse to have their leads put back on) as they're having so much fun and being home is nowhere near as interesting.
Lack of understanding - this doesn't really apply in your scenario, but it's common for dogs not to respond to commands simply because they don't understand what's being asked of them - maybe your commands are muddled or unclear, or they can't hear you because of the weather, or the reward you're offering isn't enticing enough.
In your scenario I would be wondering if your dog was feeling unwell, especially as you say she's very sleepy and hasn't eaten or drank anything today. I would keep a close eye on her and call the vets in the morning if she doesn't improve - if she doesn't drink by this evening I would be tempted to call sooner tbh.
Hopefully it's nothing and she's just tired or feeling under the weather, but definitely not something to dismiss as stubborn, especially when combined with the lethargy :)