I think she is pregnant again!
She took a Christmas photo holding Christian and seemed to pose in a way that was highlighting her belly; in her recent London photos it looks like she's wearing a stretchy maternity-like dress that also shows it off?
It is TOTALLY normal to have a belly four months post-partum (I still have mine years later!), but it seems strange to accentuate it in these above ways I've mentioned, unless it's a true baby belly!
She looks beautiful, as always, this is just my thought :)
And if she isn't - I hope this post doesn't offend her, and I think it's awesome she's breaking body standards.