With the Spring coming (don't know exactly what it's like where you live) -bugs are all starting to wake up. I found spiders around today...bees...flies...and got bitten by some little critters myself while collecting logs the other day.
It could be that she is picking up some biting insects when she's out. Chances are they will be too small to see, but can still pack a punch!
You could go to your vet and ask for an insecticidal shampoo? But of course that may not stop the bugs getting on her again. It might be better to get something from the vet than an over the counter one, as some are not safe.
In my experience, bugs that are picked up on walks (especially in grass) seem to have a biting season then disappear till the same time next year, etc. So it may not last forever!
Of course there may be another cause for this itching. At least if you take her to the vet, for the shampoo, he can take a look to see if it could be anything else possibly.
I think I read somewhere that thyroid can do this too? But I am not sure, but for some reason that sounds familiar. You may want to research that just incase.