Experiencing pelvic discomfort and sore back pain during period (2 days now). felt small, hard pea-sized lump in groin area. what could it be?

STD? : The pea sized nodule you are feeling may be a lymph node and it may be enlarged due to an infection. The pelvic pain and back pain may be due to cervix infection. Usually these things occur together in something like Chlamydia infection and/or gonorrhea. Typically acquired sexually due to unprotected intercourse with a new partner or a partner who has acquired the infection from somewhere.