Your "shy" reasoning is completely opposite of those I know with Depression. EVERYONE I know (and quite a few in my family) with depression is more of a type A personality. They DEMAND they be heard almost to the point of rudeness, talk incessantly and can be very overbearing. They are sometimes the MOST fun people to be around but can also "turn" on a dime. They aren't big on "socializing" with friends, but when they ARE around they are the center of attention. It's an odd mix of traits and sometimes hard to nail down.
One of my best friends suffers from depression but you would NEVER know it as she's hilariously funny and loud. When people find out that she has a depression problem, they are always shocked.
You are describing bipolar disorder to a T.