Okay, you are saying you were injured at work, and were offered medical treatment, but declined, right? They (your supervisor or HR) asked you to sign a paper saying you declined medical treatment? Then today, it feels worse. What sort of injury is it? Are you able to be at work? Did you ask them about it again, and now they are saying you shouldn't go to the doctor or that they will not pay for it because you signed a paper declining medical treatment last night? It's a little confusing here.
There is no way you can 'sign away" your right to medical care or worker's comp if you are actually injured at work. It will still be considered a work related injury if you want to seek medical treatment for it, regardless of what you have signed at the moment it happened.