Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
A certain level of drowsiness/lethargy is to be expected after vaccinations. A typical vaccine reaction happens within the first 3 hours after vaccination. Vaccine reactions can range from lethargy to profuse vomit/diarrhea to hives/facial swelling to respiratory distress. VAccination reactions other than mild lethargy are not common. If she is profoundly lethargic such that you can barely wake her up, then this is concerning. If you are able to get her up and she will have a treat, then you can safely monitor and hope that she sleeps this off today and will be fine tomorrow.
If she is profoundly lethargic to the point you have trouble waking her up or she seems too weak to walk or limp, then you should take her to be examined again by your vet. They should be happy to triage her and do a brief look to ensure she is okay. Or, give your vet a phone call right now.