Oh you poor thing, mastitis is awful

From someone who has had an abscess resulting from mastitis, I would say use your ultrasound referral especially since some pus has come out, just to be sure.
Or you can get some ultrasound therapy to try break down the blockage, I had 2 sessions of this before I ended up going to hospital because I was beyond any help. But a lot of people do benefit from it.

It's even more painful (and gross!) to have it surgically drained :/ I sincerely hope it will all rectify itself with the course of antibiotics