I'd suggest you search out some pics of orf to see if you think that's what you've got. If you're not sure, it would be a good idea to take some pics and post them here so we can see whether we do think it looks like orf.
Search here on orf and you'll find plenty of threads about it and the various remedies we use.
It's a virus, sheep which haven't come into contact with it before would have no immunity so could easily blow up with it. It has to enter through broken skin, so a typical situation is lambs getting it around their mouths from eating prickly thistles. Possibly the new lick has a rough rim, the new tup has deposited virus there, the sheep have punctured their lips and picked up the virus?
Being a virus, antiobiotics are pointless. I am trying to not get on my ranty horse

about using antiobtioics where there is no evidence of bacterial infection...
There is no treatment for the orf virus, the infection just has to run its course. However, you do want to guard against them getting a secondary bacterial infection, which is why people use the blue spray. In this case, the antispectic purple spray will do every bit as good a job and not add to the problems caused by over- and inappropriate use of antiobiotics.
The other country remedy which people use, including us, is to dip their faces in a strong solution of Daz. We do find that Daz plus antiseptic purple spray gives the best recovery. And yes it has to be Daz, not any of the other washing powders.
Of course if they already have secondary bacterial infection - if there is any pus for instance - then an antiobitic injection or spray would be indicated.