If I had a guess (since I've never had much to do with HITH or HLLE - hopefully I got them right), I would go for some sort of viral tumour or growth.
I don't want to suggest a med because this is outside my knowledge.
I would definitely chase up water chemistry results as mentioned above. This closes out tank water but also if tap water may have changed in the last week. (Or does your water conditioner detoxify heavy metals). If tap water is fine, then try a water change.
Also something must of set this off so I'm wondering if anything changed in your tank during the last two weeks. Other questions were if it was getting worse and if there was any pus coming out?