What can cause low white blood cell count?

I'm already stressed enough. My dad is about to have his 4th surgery in under 2 years and I'm about to file bankruptcy and I'm in a long distance relationship with my fiance. So I was getting my thyroid checked like I'm supposed to because I have Hashimoto's and my doc just calls me at work and tells me my WBC is low and he wants to test again and have me see a hematologist. That's the last thing I need to hear right now. I'm pretty sure it's not cancer but it still makes me nervous. I've been feeling fine aside from some minor congestion. What could cause it?