I'm not a doc, but this sounds like lipohypertrophy (hope I've got the right term) - it's just a build up of ordinary fat. I think it happens whatever insulin you are on - it's the body's reaction to insulin being put in a specific place again and again.
from what I understand from others who have had t1 for a long time, once you start rotating injection sites, like you are now doing, it gradually goes away. Sounds like yours is already going?
PS love the description of a 2.5 pack....!