My guitar sounds flat?

I bought a guitar off my friend that was virtually brand new. I tuned it and It was fine it stayed tuned for a few days I retuned it with my electric tuner but It sounds sooo bad. It is flat :/ And sounds diabolical, The tuner says it is in tune and everything but when I play you can tell its not :/ Any ideas of why this is? And what should I do? Thanks :)