I am not aware of lymph nodes being at the bottom of the rib cage. It would depend whether this lump was a part of the bone or soft tissue - this would involve a physician to examine you. Your description suggests that it may be a soft tissue mass.
Depending on what your physician finds, further imaging can be done with plain films or CT scans. If it is determined to be harmless, then you may be instructed simply to watch it.
Most soft tissue masses are benign - but without examining you, I cannot give any sure responses.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Thanks,
Kevin, M.D.
Thanks,
Kevin, M.D.