Under-eating??

What actually classifies as under-eating? According to my FitBit app, I'm constantly eating about 300-800 calories under my "budget" which is already set low for weight loss. And I'm exercising (mostly cardio) a lot more. Is this really that bad? I don't do it on purpose. I'm just eating better foods and having less snacks (chips, candy, pop) throughout the day...

On a related note, can this cause me to miss my period? We're TTC but I got a negative blood test when I was a week late, but I still haven't gotten af and I'm on cycle day 60 with only VERY light spotting this past week...