Postpartum/post Breastfeeding Missed Period. Any Answers Other than Pregnancy?

I am almost 7 mos postpartum and stopped breastfeeding at 5 1/2 mos. I had my fist postpartum period last month and it was extremely heavy and lasted a whole week. My husband and I have been intimate just a few times this month - without condoms using rhythm method while on the tail end of my cycle and with condoms post-period. I know you can get pregnant while on your period, I know it was a bonehead move, but we were on our first vacation without kids when I got my first period and you know how it goes. Now I am a week late. I took a pregnancy test this morning and it was negative. I called my OB and made an appointment for tomorrow morning for a blood test. Needless to say, I am FREAKING OUT. I have a 3-year-old, a 6-mo-old and can't imagine going through it all over again so soon. The nurse at my OB's office said that it was not normal for a postpartum/post breastfeeding period to be so irratic and that a week late is cause for concern.

I have not had much sleep lately and have changed my diet drastically in the last 2 weeks in effort to lose some of the baby weight. Could these be contributors?