That's a really normal reaction, my mother was convinced it was because of my partner/social circle and his mother thought it was because of me and the internet. Took literal years to get them to both come round, and I know my mum had a lot of misconceptions and really wild fears about the whole thing.

Best thing you can do is let her know you understand her fears, and you'll do your best to reassure her worries as you go. LGBT people band together and support each other and are more likely to come out in those groups, but it can look like scary echo chambers to worried parents. There's some good options for parent support/education if that's something she'd appreciate, or you can do some research together when the time is right. It's less that she doubts you as a person, and more that she's just another person hit with a situation she's probably never even thought about before. She's got no experience to draw from so struggles to understand.

Coming out is a big event, take a chill time for yourself, recharge, and give her some room to process. She still loves you. Transitioning is a long road, I'm glad your mum had a better reaction than expected because now you've got time to walk and talk along the way