Worried,
Yes, you should be worried. However, don't let anyone, especially not the numbers squirrels that frequent these types of pages (and the misc.education.medical newsgroup), automatically rule you out based on a single data point (your MCAT score).
There is a student in my class, who's sporting a class rank well into the upper half of our class, that got a 21 on his MCAT. He's not alone; I know of at least one other that earned a 21 (on his second try, no less) and is currently 300 days from becoming a doctor.
I, on the other hand, barely had a 3.0 GPA, and got interviews at EVERY school I applied to (yes, smart guy, there were several), and NEVER got a rejection from ANY school at which I interviewed.
By this, I don't want you to think that numbers aren't an important part of the process, or that I'm suggesting that you not consider retaking the MCAT.
All I'm saying is that numbers don't tell the whole story.