I started to feel "bigger" at 5wks - dont think this was just in my head as my pants started to "shrink" and belts felt like "in the way" ... Its my 3rd child and I am 36 - I think the changes taking place in my body comes a bit faster that with my first two pregnancies? Oh yeah ... and its been years since my last pregnancy - 14years to be exact!
I'm 9 weeks and I already gained six pounds so your definitely not alone.
I gained weight right away with my first..gained a total of 30 pounds. I was one pound away from pre-pregnancy weight when I got pregnant w/ this one 9 months later...I haven't gained any weight yet and I am 8 weeks..but I have been sicker this time, too...ugh. Oh, and I have the same due date as you! :-)
With my first pregnancy I was showing at about 3 months this time around I was basically showing at 5 weeks. So for me it is normal to gain early while others are lucky and start gaining later.