I am sorry to hear that more masses have appeared and that some seem to be fast growing.
What does your Vet advise re removing the masses ?
Removing them before they grow too big might be a good idea, especially the one near a leg joint. Sending them off for histology would probably give a more definitive diagnosis than the small samples originally sent. But a definitive diagnosis may not necessarily offer any treatment options.
I'd ask the Vet if removing the masses before they grow too much more might be the way to go, regardless of what the masses actually are. The bigger they grown the more tissue would need removing and the potential of the mass near a leg joint causing additional mobility problems may be something to consider too.
But of course my opinion is unqualified, your Vet may completely disagree.
Sending lots of vibes for Herbie xx