I've had a lump under skin of my left hand for few weeks. it's firm but squishy. doesn't hurt but hand sometimes hurts. could it be a fatty deposit?

Need to remove it: It could be any number of diagnoses, including a synovial (join-derived) cyst, a non-synovial (hair follicle) cyst, lipoma (a fatty growth, but rather uncommon on the hands), or even an abscess ( a localized infection). I would remove this growth. A dermatologist/dermatologic surgeon or hand surgeon can do this procedure and once it is removed and biopsied you will know the diagnosis as well.