Liz Buchanan BVSc MRCVS
Hello. This sounds like a pyometra - they do sometimes happen in younger female dogs - and even if not, vomiting and lethargy are red flags for dehydration, so it would be much safer if you took her to the vets as an emergency today. If this is an open pyometra, your vet may decide not to operate immediately, but at least they can assess and treat for any toxaemia etc. It could also be skin disease or vaginitis, which are much more local infections, but the more general signs of illness may still need to be addressed.