Re: Strain? Tear?
I too, am far from a doctor and unfortunately I'm starting my "Treatment of Athletic Injuries" class next week, BUT I am a rugby player... wait, that probably makes me a less reliable source doesnt it? Hmm...
Anyway, if it's healed in such a short amount of time, it was probably just a slight strain. As far as prevention goes, it may be a muscle imbalance as you suspect, or it could be a matter of form. I'm not sure how large a part the pecs play in pullups, but being a pulling motion I would imagine they act mostly as stablizers. Either way, work on strengthening your chest once its fully healed with wide pushups and dips to prevent future injury.