According to your saying, it sounds more likely to be a lipoma
, however, you can do the ultrasound
of soft tissue
in that particular area to see what's going on.
Based on the findings of the echo, it would further proceed: if you really need a surgical remove of it or "watch and see" solution or/and if you really need an MRI.
Most probably that mass can press the nerves passing in that area and causing stiffness
and soreness of the finger.