Mast cell or allergy?

5 year old weim has had 2 small mast cell tumors removed over the last 9 months(hind leg and abdomen) both were noticeable. Then all of a sudden this popped up on her ear. Skin was dry and the hair came off area as I was gently examining the area. Hasn’t changed in size in the last few days.

Hello!   I'm afraid that this is a bit of a  tricky issue.  Nobody can tell you, either online or real life, that a lesion doesn't have mast cells in it from the external appearance.  Furthermore, a lot of allergic reactions are actually thick with mast cells.  In my early years as a vet, I once diagnosed a 'Mast cell tumour' (using the recommended methods at that time - fine needle aspiration and sending it away) that turned out to be some kind of inflammatory mast-cell foreign-body reaction which disappeared on its own (luckily the owner was delighted rather than annoyed).    Nowadays we know better and are much more careful about how we interpret mast cells found on cytology (I trust that mast cells have been found?  I have just realised that I am assuming that context from your question).  If mast cells haven't yet been found / the lesion hasn't been sampled, then the list of possibilities may be much broader than "mast cell tumour or inflammatory reaction" so testing may be a good idea; if mast cells have been found, then have a chat with your vet - who may in turn have the advice of their pathologist - as to where to take this from here.

Having read the context again, it sounds as though this lesion hasn't been sampled at all at this stage.   Therefore there is a wide range of possibilities, of which mast cell inflammatory reaction and mast cell tumour are only two possibilities.  Your vet may want to test for ringworm - so please do not handle this lesion without gloves and wash your hands well.  Foreign body reactions, demodex mites and various others are also possibilities.