I have that problem since I was young. I hardly speak, still don't speak much (I'm close to 40).
When I had to answer a question in class, it would take too long. Sometimes the teacher suggested I had not studied the material. Some teachers understood better that I have trouble reproducing information. I work better with recognizing information, not reproducing it at cue. When I talk I prefer short answers. Mostly I prefer to talk to some specific people, because they like to talk total nonsense, so I don't need to worry about stupid things (I seem to say stupid things quite often).
Some days I am in a "talkative mood", some other days I can hardly speak. I also have days that speaking is difficult, like my mouth is frozen.
Also in my case people would say that I am shy, later on it would be called depression, social phobia and generalised anxiety. Pretty useless labels though...