I posted a thread a while ago about this...I was on Othrotricyclen (if that's how you spell it) too and when I started Atkins I had the same problem. For me, I started Atkins at the beginning of my cycle and after two weeks, my full blow Tom came to town. To make a long story short, of the first two months that I was on Atkins, I had TOM for 6 weeks. It was horrible and expensive Anyway, I went to the Dr. and they didn't know what to say, but they put me on a higher dose and I've been fine.
I couldn't get the Rn I saw at my Health Center (I'm in college) to tell me whether or not the pill was less effective (due to all the spotting), so that's why I changed my pill...no babies for me, not yet anyway

You can do some searches about this on the site, but what the "general thought" is that since your body is releasing more fat cells (and that's linked to estrogen some how) it can bring on your period more frequently. Definitely do a search, I will and I'll reply another post

I'll be back with a link,
Erin