Hi everyone

I really need your help and advice.
Let me explain my situation for those who don't know me I have two yorkies, Twinkle (female 1 year) and Wicket (male 8 months), and I recently lost my mastiff, Kaddie. I've worked really hard since she died to keep Twinkle and Wicket's routine the same as it was before and I think they have slowly come to learn that she's not coming back
Twinkle has become gradually more 'grumpy' over the last few weeks....she is less tolerant of Wicket playing and has snapped at him a couple of times when she's playing with her toys or eating. She has
never done this before, but I put it down to her feeling a bit confused over Kaddie. Me and the children are able to take toys from her, and I can pick up her food bowl while she's eating without her snapping or growling, so at first I figured she had an issue with Wicket.
Then over the last week or so she has started growling or barking at me, really aggressively. I can be doing nothing in particular - sitting on the sofa, cooking dinner etc and she just starts doing it. She looks at me really intensely and this low growl begins and her teethe are on show.....It's really quite scary.
A couple of days ago Twinkle and Wicket were playfighting in the living room and when I broke them up (they were very noisy!) Twinkle snapped at me! Her ears were back and she did a really low deep growl!

and then she actually went to bite me! I was completely stunned as she has always had the sweetest most friendly personality.
Yesterday evening was the biggest shock to me though - I was sitting on the sofa, Twinkle and Wicket were curled up on their bed on the floor. My daughter climbed up on my lap to read a book and Twinkle just launched herself up at my face - from the floor! Again she had her ears pinned back and was growling, her teeth were bared. She looked like a different dog! When I told her 'NO' she began barking at me really aggressively. I have no idea what triggered it.
I'm completely at a loss how to handle this. She hasn't shown this behaviour to the children, but obviously I can't take that chance. This probably sound stupid, but it's like she suddenly hates me

Maybe she feels confused that Kaddie isn't there and is taking her aggression out on Me and WIcket?
Should I take her to the vet's incase there is an underlying medical problem? Or is that not likely? Is it worth getting a trainer in? I want to nip this behaiour in the bud as soon as possible.
Sorry for the long thread....but I'm so so upset over this.What has happened to my sweet loving dog? I was in tears most of last night......I feel so guilty that my poor baby is going through something that is making her act this way - and I don't know how to help her.
I'm grateful for all advice and comments - thankyou in advance.
(Extra info: Twinkle is spayed. she gets 2 30-45 minute walks a day)