Muscle injury likely
Assuming you mean in the lower-mid outer abdominal area, then a strain of an intercostal muscle (between the ribs) or a partial tear of an oblique muscle (outer abdomen) would be the most likely cause.
A rib fracture
is possible, but less likely as more force is typically required to injure a rib. Treatment is usually symptomatic and includes rest, ice and anti-inflammatories.