I've missed my period this week. I'm considered underweight and it's possible I have hyperthyroidism, but still this is very strange for me, and in the absence of the severe, crippling menstrual cramps I'm accustomed to, I've been having this sudden feeling of a heavy rock sitting in my stomach. Its possible I could be pregnant but when I noticed yellow urine I thought of dehydration, and this past week I had a shift in schedule, which I've read can contribute to a late/missed period. The only other signs of pregnancy I've experienced are fatigue and heartburn. Is it more likely that I just missed my period due to temporary conditions?