"Everything hurts" in the context of having had a real sickness not long before made me think of post-viral syndrome (I think you may call it chronic fatigue syndrome in the States). Alex is a little young for it AIUI, but I've known a girl who developed it at the age of nine. However, your explanation is also plausible, especially if Alex doesn't seem unusually tired. I think the main thing we did to prepare our kids for the arrival of siblings and how it would affect me/my time with them was to offer lots of information about what would happen when I went into labour, what if the older kids weren't at home when it happened, was it going to hurt me a lot, etc.