Looks like scab to me, FiB.
You did treat all your sheep with the Dectomax?
I think you've said that you have shared boundaries with (an)other sheepkeeper(s)? You must tell your neighbours what's happened, and if your ewe gets better with the Dectomax that it looks as though it was scab.
Unfortunately, both scab mites and chewing lice can survive away from the sheep for 17 days, so your fence posts can remain infective to your neighbours' sheep for that long. And, if not treated soon, your neighbours' sheep will be able to reinfect your sheep once the Dectomax has left their system. Withdrawal period for sheep is 63 days, so you want your neighbours all treating their sheep within the next month, really.