My girlfriend is growing breasts without estrogen? Is this even possible?

Hi so I'm actually ftm but my girlfriend and I have a question. Recently we've noticed that her chest has gotten bigger (grown to an A cup) and her nipples are getting bigger (probably tmi but they went from penny sized to the size of a loonie, if you're Canadian, or like the size of a bigger coin). She isn't on any estrogen or blockers, and I know she isn't taking them behind my back (because why would she? I'm very excited for her to start medically transitioning! But also her family isn't really trans friendly and that's where she lives for now.)

Last year we did get some bloodwork done because we were having issues in the bedroom, and her testosterone levels were low. So she got prescribed testosterone, she took it for a little bit but we stopped and we're good now without the hormones (and they didn't really help all that much).

I'm guessing this is probably because of the lower testosterone. We were just wondering if this is anything to worry about or if anyone else has had this happen. She's also gotten more curvy and her skin feels smoother. We like to joke that her body is forcing her to transition 😅

I'm sorry if I overstepped a little, I know that this is a place specifically for you lovely ladies (and lovely people!) But me and her are curious and doctor google only gave us the diagnosis of gyno (which it could definitely be). What better place to ask than here? And if we went to a doctor, they'd probably recommend testosterone because "mEn dOn'T waNt TiTs" which is true for most guys, I'd think, but my gf isn't a guy lol. Sorry for the excess brackets as well 😅