Small smelly wart-like object on cat (15y)

My cat is 15 and and has had what I can only describe as some sort of wart looking bump on her skin for years and years. However 4 days ago I noticed it had freshly burst and there was a small amount of very mad smelling pus coming from it. 2 days later it had scabbed over black and some of the pus had turned black as well. It still has an odour now and my cat seems to be slightly sleepier than usual however I’m not certain whether that is me just imagining symptoms. Would really appreciate some help. Attached are the photos first: from when I initially saw it and second: 2 days after and how it still look at present. 

My old cat had something similar and it turned out to be infected. Better to be safe with a 15 year old kitty.

It could be an infected cyst, abscess, or another type of growth that has become infected

Hello it sounds as though there is a lot of of sweaty material gathering at the hair follicle instead of being released onto the skin as usual.   There may be various reasons for this including a papilloma, localised infection or demodex, or even an underlying immune concern allowing this to progress, so a vet appointment is a good idea.