I'm guessing that's what it is.
Went out on my lunch break to take her rug off, and straight away noticed a big lump on one of her 'breasts'. Hoof shaped area with hair missing though no broken skin I could see. Not really sore to touch but didn't like me poking it much!
I guess I just leave it for a while and see if it goes down? She appeared to still be walking fine (As well as I could tell while hobbling through muddy paddock in high heels, trying desperately not to lose one!).