I have a 3 wewk old chihuahua I have been caring for since birth, mom died having complications,she has been doing great but today she vomited and now she is sneezing a lot with mucus coming out what can it be and is there anything I can do for her?

Have you seen a vet?

At 3 weeks any sign of illness must be seen to by a vet asap. There's really no way we can give you a diagnosis over the internet.

Good luck.