My vet prescribed cefotaxime for my bp. He told me to give it under the skin, but I am hearing you should administer it into the muscle. Anyone have any experience or knowledge on this?

I ask because her 3rd shot has left a slightly swollen spot. Its been about 7 hours and its puffy and slightly swollen. Its not the lump of medicine, which i saw after injection. That dissipated quickly. I swap areas each injection. Any thoughts?