Hi
Welcome to the MedHelp forum!
If the lump is under the skin but over the rib, it can be a collection of fat-lipoma, or connective and nerve tissue or neurofibroma or even a cyst (either a glandular cyst or a bone cyst).
If it is under the rib, then it can be due to gas, hiatal hernia, spleen abscess, or a hernia.
It is difficult to comment beyond this on net without examining the lump. Do discuss this with your doctor and get yourself examined. Take care!
Sounds like it is your lymph node or your spleen that you are feeling. Localized lymph node swelling generally means some type of infection is present. If it is tender or does not go down in next few days, you should go see a doctor. If you are not having any other symptoms, the doc may want to do an MRI on it.
Hope you feel better.