Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concerns..
As per your complain it seems that a red and hard marble sized painful lump in the groin area at the underwear line can be due to hair follicle infection
It can also be due to infected and swollen glands in the area..
At times infection over the skin can also cause inflammation and painful swelling..
I would suggest you to start applying a combination of antibiotic and steroid
ointment over it..
Do cool compresses atleast 3 to 4 times a day to reduce inflammation and pain..
Take Anti inflammatory painkiller like Ibuprofen
You should wear loose fitting undergarments to avoid friction in the area that can further increase infection..
In case if it still does not improve consult a General Physician and get evaluated..
Hope this information helps..
Thanks and regards.
Dr.Honey Nandwani Arora.