I found my dog shaking this morning and his legs were stiff. Has anyone had this problem before?

My dog Lady is 8 months old and I noticed when she played too hard, her head would shake and she would get really lethargic. Used to think she was getting sick so I would give her a tylenol and she would be all better the next day. Usually happened towards bed time maybe 4 times her whole 8 months. Today was different she was playing really hard with her brother late last night and my son said her body and head was shaking and didn't tell me. That's when I found her this morning shaking really bad. She won't snap out of it and now she's not eating or drinking. She did finally eat some chicken broth but left the chicken. I just know she don't have parvo or anything like that. I honestly believe she has a heart problem or has seizures. The vet can't see her until tomorrow morning and she does look somewhat better but has been sleeping all day. Can anyone tell me what they think because tomorrow is a long way off right now and I live in a small town? Thanks!