Having the chills

This last few days I've been feeling a bit off-colour. For one I've been feeling cold even though it's been a mild 20-odd degrees and hasn't been raining. It's the kind of cold that goes deep into your bones. I've also been feeling lethargic and tired all the time although I think I have already repaid my sleep debt. I had a good 11+ hours on Friday night after an exhausting day out in town. I don't know what's wrong with me. I don't have flu symptoms, nor a fever, as far as I can tell.