Ceiling White Brown Spot Question

Hi all

I'm doing a DIY paintjob on my laundry. I was rolling the first topcoat on the ceiling (Dulux Ceiling White Flat) yesterday and noted two things.

From the start of the roll, the paint was bubbly and it took a bit of rolling for the bubbles to disappear from the ceiling.

Second, about half way through the roll, some red-brown patches about 5cent size appeared in a small area I'd rolled. (This also happened in a job last year and, since that tin had been almost finished, I threw it out and used new paint.) This time, I just kept rolling and the patches eventually disappeared and no more appeared. Anyone else had this happen with Ceiling White (or any interior acrylic paint) and know what it is?

Details:
Roller: Over 1 year old, 230mm composite fibre 9mm nap.
Ceiling White: 4L tin, opened 2 July 2014, 2/3 used.
Ceiling Surface and Preparation: Plasterboard with existing ceiling white (painted ca Dec 1993). "Wounds" sealed with Dulux (Oil) Prepcoat Sealer-Binder, filled with plasterboard filler, resealed with Dulux (oil) Prepcoat Primer, Sealer, Undercoat, and overall undercoated with British Paints (Acrylic) 4-in-1 White.

My own thoughts run to: (1) Rust (some noted along part of rim (and possible on seams I couldn't see), although I cover the rims with alfoil to limit paint-rim contact); (2) Unmixed medium – the paint had been sitting for about a year, and with one of the paints I've been using the medium had separated although I can't remember now if it was this tin; (3) Paint "going off" (got no idea what that means but from some tins I've smelt I know it does go off!); (4) A peculiarity of Dulux or any Ceiling White.

Many thanks,

ausgumbie