Friendly hamster suddenly attacked me?

I got a lovely hamster not too long ago and she's a few months old. She has a nice temperament and always begs me to hold her and let her out of the cage.

Today, she was awake so I opened the cage door and she climbed out and onto my hand as normal and took her to sit down and I'm stroking her until suddenly, she makes a weird noise and before I really know what's going on she has moved lighting fast and clamped her mouth down on my thumb really hard. In shock I shook my hand which I shouldn't have done because it threw her off over the edge of the bed into the wall (which I feel incredibly guilty about)

She's always been nice, I don't understand, she has had her quirks but I never expected this.

Is the cage too small? The cage I got for her is the pink rosewood cage that has multiple floors.

Other thoughts is I recently found out she's in something called 'heat', does that affect anything?

The only recent change I have given to her is because of the pandemic I havent been able to get her a ball they were out of stock everywhere, recently I got her one and put her in it for the first time last night but this is the longest I've had a hamster without putting them in the ball. She's handled regularly.

I just want to know why she's like this and if its something I did, it's really upsetting thanks.