Hi and welcome to YT

.
So sorry you're having to deal with a scary issue. It's so tough to say whether the new lump is also mastitis...does it feel like it? And yes, theoretically, if she is still on the meds one would think that any add'l infections would *not* develop here.
I'd try not to worry too much at this point, and make an appt to get back in ASAP.