My dog was just diagnosed with histiocytosis?

I noticed bumps on her head so I took her to the vet. They said she had histiocytosis and hey gave me some pills for her and some ointment too. They look soo much better, but I've noticed she's gotten a couple more on her ears. Is it normal that more or popping up? She said it could take up to three months for them to go away but idk if it's normal that she's getting more of them when she is still taking her pills.

Yes it is normal for her to get more even while on the medication, since it is normally caused through auto-immue function the medications are only helping the current histiocytomas, others may arise during treatment. Do no panic, they will typically regress on their own within three months, I would also contact your vet just to inform him of the addition growths.