Advice please regarding a dog.

I have a 2.5 year old girl pure pedigree boxer dog. My SO rescued her from a building site where she had been dumped. She was around 8-10wks old, full of flees, mites and mange. We took her to a vet imediately and paid for her to be sorted and cared for. We took her home fell in love and started to build her up nutricianly and with lots of love and attention. Shes always been a very docile and placid animal. Never ever growled before. We have a kitten too and they get on ok. Shes always been obediant. Today i was getting up to make a cuppa tea and she charged at me knocking me into a wall then she went for me! Growled, snarled and actually pysically went to bite me! I luckily managed to gain my balance and get the other side of a gate in time. I was absolutely beside myself in tears petrified and imediately i muzzled her for my own safety. We only muzzle her on walks as she eats rubbish off floor when off the lead and last time she nearly died after eating a posioned piece of meat. Advised by vet to do this. We imediately remove the muzzle when shes on lead as we can see her and watch her and when off walks. I have no idea what triggered this off and since muzzled shes growled a further two times nastily. Im 33 wks pregnant and she hurt me knocking me today but baby has moved fine so i think im just bruised and will keep eye upon things. What the hell do i do with this animal?? Im totally scared of her and lost trust. I have a 11yr old daughter and baby on way. I cant risk a vicious dog! But i love her to pieces and im beside myself shes done this. Im thinking to get her checked at a vet making sure its not a medical issue firstly then if not do i say ok you've snapped twice i have children sorry you have to go? Or what else can i do?? She is still my baby even though im pysically scared right now of her? Please help any advice would be greatly appreciated. Thanks. Sorry if this is  in the wrong group.