Toni, how are the D above it and the Bb below it sounding? If they are both perfectly in tune (with a tuner of course) and the C is still flat then you might have a couple problems. You might want to check the first pad on your RH ring set. It might be a little too low thus not allowing the right amount of air to escape and making the pitch flat. Also check all of your springs to make sure they are on the right way and that all of your keys bounce back when you tap them. Check to make sure your middle joints are securely together and they don't wiggle (you might have a cork that doesn't seal all the way...thus allowing air to escape. I don't know how old your E11 is, but when I was playing on mine in school it was made out of a mix of grenadilla wood and ebonite. Your wood could possibly have warped somehow or split somewhere near the C hole. If all of these things check out...take it to a repair technician you trust. Good luck!

-Dennis