negative pregnancy, but I'm late

I'm late by a week, I've taken a test but it was negative. I have irregular periods ranging anywhere from 4-6 weeks, however, I've never had a pregnancy scare before. It's been five weeks. I've gained 8 pounds though. Two people have said I look pregnant. It's a little concerning since I've always weighed between 98 lb to 100 lb, but now all of a sudden I weigh almost 110 lb. What should I do?