What could be the cause of enlarged lymph nodes before and during the time of my period? recurring with sore lower back. genital herpes?

Probably not herpes: Herpes rarely if ever causes lymph node inflammation without an obvious outbreak of genital area blisters, sores, etc. Even then, lymph node involvement is rare in recurrent herpes, only during the first infection. Finally, genital herpes doesn't cause back pain, and rarely recurs as often once a month -- usually no more often than every 2-3 months. Discuss w/ your doctor if the problem continues.