JFB I also reckon they start to pick up on the fact that things are different with you and it's almost like they sense you'll be taken away from them soon. That was my experience with both girls they became ultra clingy in the weeks and days before I gave birth to Scarlett (Lucy was the clingy one then) and with Marc (only Scarlett).
I have bad mummy days where I just wish they'd leave me to cruise the internet all day but thankfully they don't last. We wound up having a great day today - we've been spending heaps of time at the park (nothing like having a puppy that needs exercise to make that happen) and it's doing the world of good for all of us - I dread the rainy weather as that'll disappear and we'll be shut indoors all day. Yuck.
MG been there and it's rough