Glad you had a cardiac work-up. I would have recommended it if you hadn't already done it. Your heart is obviously fine, so welcome to the world of heart palps! Very, very common for those of us with anxiety/panic and 99.9% of the time, totally harmless. Scary? Without a doubt.
Being of a well seasoned age, and very happily past the monthly mess business, I'm afraid I'm not the one to address this quesiton. I'm sure there are other, younger women here who will respond with their own experiences. In the meantime, I strongly urge you to visit the Heart Rhythm Forum. There are many wise womenfolk there who can no doubt answer your question. Not much they don't know about wonky hearts.
I'm wondering, and this is just a guess, if the additional hormones during your cycle have anything to do with the increased palps? Hmmmm. Not a bad question, if I do say so myself!
If you can't find an answer that calms you down, please see your OB/GYN and let them know about this issue.
Peace
Greenlydia