Sciatic nerve and deep pain in legs a week before my cycle?

Need Evaluation: If that occurs once or very rarely i would not be worried. But it is continues, it may be a sign of something affecting or pushing against a nerve. For instance endometriosis or a fibroid. So if persistent you should be evaluated by a doctor.

Endometriosis?: This is concerning and i would wonder if you might have endometriosis with implants in the sacral area that react with your periods and are causing some nerve root irritation. I would recommend you see your pcp/gyn and have a good exam and possible other studies to see what is going on. May need and ultrasound or even ct or MRI of the area to see what is going on.