I have been having pain much like a muscle being caught in my ribcage about 3 inches under my left breast. What does this sound like?

Many possibilities. Depending on age, medical condition, activities, it could be many things. Muscle strain, even with "minor" trauma could do this. A cracked rib is possible. Rarely it could be referred pain from the neck or mid back. If it worsens or continues, see your primary provider for an assessment.