I do not think the oral contraceptive
is the cause for your lump. A better way to diagnose this would be to take a fine needle aspiration biopsy
(FNAB) of the lump and to study it.
There must be a chronic systemic inflammatory condition, which is causing the lump. It could be enlarged inguinal lymph
nodes as well.
Please get the above investigation done. I personally feel there is no need to change the contraceptive.
Wish you good health.