I'm 18 weeks and have only recently started to show. Woke up one morning and boom there was my bump. I also haven't felt the baby yet. Went to the dr yesterday and heard the heartbeat and all is well. Just be patient. I know it's hard. Trust me I was getting nervous too. 

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. 

I was about 20 weeks before I started showing, and didnt wear maternity clothing untill about 25 weeks, and most clothing still fit way farther into my pregnancy than that.

 I felt my baby move at about 18 weeks, and subtle at that.

So don't worry! As long as your doc says everything is ok, you are fine!

(I went up 2 cup sizes by week 20, I am 7 weeks PP now, nursing, and still that same cup size)