Experiencing swollen lymph node in groin and sore back pain during period. what could this be from if no visible infections ? when to be concerned?

Lymph node: If the lymph node remains swollen please see your physicians and get a definitive diagnosis

Infection: that is not visible. Nodes are very common in this area and if they are not very big (under 1/2 inch at most) then, just because the area has germs and can get chafed is enough for some nodes. Internal infection, or infection in a fold of skin can cause infection (often MINOR) without much b eing seen. overall viral infections can cause this (like colds) and the nodes last past the illness.