Cat with symptoms after vaccination

Question
My 2 year old male cat, Dick, received his yearly booster on Thursday evening. Beginning Friday a.m., was beginning to show discomfort, moving slowly, by Friday evening, showed lack of strength in rear legs, would take a few steps, low to the ground, and then sit down. Took him to emergency clinic, they checked pulses, (good), indicated he was not experiencing pain, although gait looked painful. Put him on prednisolone 5 mg 2 X a day. Dick was seen on Saturday by his doctor, who did an x-ray on lower half (found nothing), and also checked blood, kidney, etc., also found nothing. Have you seen this muscle(?) issue in vaccinated cats, also am wondering if lymes disease might show this way (we live in a high lymes area). After 2 days on prednisolone, there is very minor improvement, but he still is moving a few steps at a time, missing litterbox, etc.
Thoughts, and what would you do for next steps if you were me? THANK YOU!

Answer
I would suspect that if this just soreness from his vaccines, the prednisone would have already helped.  Soreness is typical after vaccines, especially if rabies is given in the back leg muscle, but should only last a few days.  Yes, Lyme Disease is a possibility and we have seen it in outdoor cats and there is no vaccine against it for cats.  Lyme Disease will cause joint swelling, pain, enlarged lymph nodes, fever, and general lethargy.  I would recommend having him tested.  If your vet has the canine 3DX heartworm tests from Idexx, this test can be used to test for Lyme Disease in cats as well...results in 10 minutes.  Lyme disease must be treated with antibiotics for weeks and will not respond to other medications