Don't panic!
6*18mg = 108mg. While this is a pretty high dosage, it shouldn't be enough to really harm you. If you don't have very much tolerance, it will likely be uncomfortable and anxiety producing, and the crash will totally suck.
Just try to remember that you have not taken enough to hurt you, and you will be okay after it's worn off. I think the risk of panic attack is your biggest concern (panic attacks are no joke btw. It is very scary to be truly convinced that you are about to die, or suffer permanent injury).
If you have a benzodiazpeine on hand, I would take it. If not, just ride it out, and don't take anymore uppers (coffee included). Some people actually enjoy taking this much Concerta.
good luck.