That happened to my dog. She was 4 and a half months and over 2 days she wouldn't eat much food and she was very lethargic and seemed tired. I found out that she had an upset stomach from a pig ear because she vomited, her poo was sloppy and when I gave her another one (on the 2nd day of sickness) she dug a hole and buried it.
I was also very worried because these were tick symptoms. Is she vomiting? I'd check her fur to make sure there isn't something tasty clinging in there. Taking her to the vet is a good idea, they'll find out what's wrong. Hope she gets better.