An injection will not help. She needs to remain on antibiotics for several days if there is an infected surgical site.
Also, no vet will authorize the use of an injection over-the-internet for fear of problems that could arise out of complications with its administration, reactions, etc...
it's just not worth it, when you can go into a vet and get a cheap oral antibiotic.
Certainly, cleaning with hydrogen peroxide or diluted betadine is fine, but she will really need an oral antibiotic.
Also, you'll have no recourse with your vet if you don't inform them and give them the opportunity to fix the problem.