OK, first rule of the internet - the words aren't worth the paper they're printed on. There is good advice on this site, but if you are concerned, there is NO SUBSTITUTE for going to see a vet.
If your pet's behaviour has changed suddenly, this means something is wrong. It might be something small, but only a vet can tell you.
From the example you give though I would guess (And I'm not a vet, I'm just some guy on the internet who also had a dog he cares for):
Your dog is worried about something happening. This is usually based on a 'cue-response'. Such as, for example, having eating something (like grass) and feeling a bit uncomfortable passing it.
Your dog is having problems with eyesight or sense of smell, and so is having a harder time recognising you.
But basically - you know your dog, and if it's behaviour has changed suddenly, then 'something else' is wrong and if you can't tell what that is - a vet is the person who can.