Winter and Charnley's The Sheep Keepers Veterinary Handbook has this to say:
Abscesses - can be found on any part of the body......if they appear around the head and neck they should always be a cause for concern.
1 - Caseous Lymphadenitis (CLA), a disease which is not amenable to treatment
2 - Actinobacillosis, a sporadic disease which can be treated
We had a young tup with what looked pretty much like your sheep's lump, which turned out to be CLA. We spent weeks cleaning out the thick, solid-looking pus and giving LA antibiotics all to no avail. In the end we had to call the disposal man with his captive bolt gun.
I have no idea what causes CLA, or if it's catching, but it sounds as if a consultation with your vet is in order.