I'm 25 weeks and unless you saw me everyday you wouldn't have noticed I was pregnant until about 23 weeks, I looked like I was just thickening through the middle a bit. I'm just now consistently having a bump. In the mornings there's not much of a bump, but then at night I'm all belly.
If you're really nervous about it, call your doctor to alleviate your anxiety a bit. On a side note, I bought my first maternity clothes about a week ago. I only bought shirts because I've been using a belly band so I can wear my pants a little longer.
I did read at some point that people who are more physically fit might show later than others. Before I hurt my knee (right before pregnancy) I was running between 5-10 miles a day, and I had to stop for a while due to the knee, and then due to the little guy. But that's what I attribute not showing right away to.