To be honest - I would say one of those comments, just because it's an unusually close age gap in our western society. And they are all correct, it's about me not you! People forget that they aren't the first person to make a comment (eg. like natalie --> gee your hands are full - gosh no has ever pointed that out to me before!)
Just reply to every single comment - "Yep. And we are so happy about it" - that puts it right back on them.
I'm sorry it's upsetting you, it could be hormones? you've still got ones from Chelsea's birth swimming around? Hang in there, lots of support at this site, and of course you can always vent in here and get it off your chest!
I'm really happy for you btw - Congratulations!!