Not really uncommon. I prefer to be vocal most of the time and even tend to rant. However, sometimes I just don't have reason to speak and go by without speaking much for the rest of the day. Once, I went a week without much words other than "Thank you" and "Hello". Not because I was mad or anything. Just didn't feel I needed to speak. I'd go for a swim, read a book, or just sit outside and relax. Just had silent days.
Sometimes I enjoy the silence. It kinda depends on my mood. I'm naturally a friendly person though, so I can't stay silent for very long. Of course, I'll occasionally let loose and act crazy when I'm surrounded by friends.
There's also a difference between acting stupid and having an actual conversation. A lot of people like to make small talk just for the sake of it ("so what you up to?"), which is fine but isn't really much beyond small talk. I prefer to think before I speak (most of the time) and not act irrationally. As a manager of a town, I have much to do for it so I kind of have to be vocal. Some days though, I just let things run their own course and just observe or play in silence.
When I'm not playing games or listening to music, I like to read sometimes. The silence leaves me alone with my thoughts, very much like the thoughts that race in my mind before I go to sleep. I'd say it's normal and no one should tell you any different. Human beings are very social creatures and their whole existence has evolved around the ideas of communication and teamwork. But we're all different and we all think differently. Some people find comfort in just keeping quiet and observing rather than making gossip and small talk. They aren't shy or introverted. They're normal.