i had this feeling when ds was around 9 months old, I think the realisation that you will never be pregnant or have a new baby again just sorts of hits you, and as it has been a part of your life and your planning and mentality for so long it is kind of a hard thought to get your head around!
personally i have 3 kids, i think the jump from 1 to 2 was hard but not from 2 to 3!
I had this from about 4 days after DS2 was born til he was about 6 months old but I kept fighting the urge in case it was just hormones. DS2 was/is a tricky baby so I have no idea why I was so desperate to do it again! He's 14 months now and I am so glad we didn't go for it. DS2 has definitely completed our family and now the hormones have calmed down a bit I definitely have no desire to have any more children. I would say be cautious, 5 weeks is very early days. Having said that I am sure if we had have gone for it and conceived then we would have been thrilled. I am just glad at how things worked out. If the desire to have another doesn't go after a year then crack on ;-)
Is it because you have two fairly close with a common age gap, and then a larger gap before your 3rd? Maybe you subconsciously feel you need to balance out your family and provide a playmate for your 3rd? 5 weeks is very early to be worrying about it anyway!
I had this need for dc4 when my ds2 was hours old, it lasted for months and months then I kind of accepted that 3 was our fill, though never agreed with it. Dh wants to stick at 3 and as he is the main breadwinner what he says goes as far as he's concerned. (Source of irritation for me if I'm honest, him thinking I do nothing all day and my job not being good enough! I work in his mum and dads sandwich shop and I love it.)
Then it came back!! Ds2 will turn 3 in July and has been the hardest baby/toddler out of all 3, horrendous tantrums from 9mths where he bangs his head on the floor/wall/table/chair leg... I have never ever been glad that he was our last baby.
My ovaries ache, it would be utterly useless trying to talk to dh about it as he gets quite angry whenever I mention that I miss our children being babies. He thinks he did me a favour by agreeing to dc3 because he could see how much I really wanted him! I will not try to convince him this time though.
Sorry I didn't mean to waffle on. It's hard not to sometimes when it's something I can completely relate to but can do nothing about.