Hi,
I remember wondering about this too. I was so bloated early on that I felt like I was showing and even suspected I was having twins, gained about 10lbs in 8-10 weeks.
Once I got beyond the 1st trimester and was no longer gassy and bloated I began to workout and feel "normal" again. We didn't go public about the pregnancy until 4.5 months, but even then people who I thought suspected at work said they never would have known and that I was "barely showing". Not true, but people are looking for a giant bump, if yours is subtle you probably look like the average woman with a chunky middle. It's pretty common, even in young women who are otherwise slim. I began to see EVERY midsection and thought so many people must be pregnant...but that's because I was comparing everyone to my "before" photo with a flat tummy.
I truly began to "pop" about 2-3 weeks ago too, like Manmohini. I'm 27 weeks now and it still depends on what I wear, some people on my floor at work are giving me the "is she or isn't she" look at work. It's funny, and sometimes I wear very obvious maternity style tops. Go figure.
The comments about being too small are annoying, they made me worry a bit. But try not to let it get to you, people love comparing you to ANY other pregnant woman...themselves, their sisters, girlfriends, etc. I went grocery shopping and an old woman at the cash register asked if this was my first. I was thrilled!!!! That was 3-4 weeks ago. She was the first person in public to ask about it and have confidence that I was in fact pregnant and not just chubby.
For what it's worth, I think I started feeling pregnant and not chubby at about 5 months. My midsection became more firm and curved and these "thick" looking. So you may begin to feel that way any day, depends on your body. Embrace it.. or HIDE it until you begin to really feel preggo. I enjoyed the fashion challenge of having to "cover my secret" at work...it was kind of fun and there are soooo many empire waist tops and boxy button downs out there that are cheap.
Good luck to you and your bean, enjoy the evolution of your body, it's a wonderful thing and you ARE beautiful!!!!
Steph