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Hello. Thanks for the question.
It sounds like she may have a fever. That is often a cause of lethargy and weakness in dogs. The fact that she's still eating and drinking is good. If you want to check her temperature, you can use a digital thermometer and take a rectal temp. Normal is 100-102. > 103 is a fever.
If she does have a fever, then I would get her seen and started on antibiotics. If the temp is normal, then I may try some anti-inflammatory meds in case she's painful. If you want to try something at home, you can use Aspirin. The dose in dogs is 5 mg per lb body weight every 12-24 hours, as needed.
I hope this helps, let me know if you have any other questions.