I put on 8kg throughout the pregnancy and lost 13kg within 8 days. But a lot of that was fluid (my ankles rivalled those of an elephant by the time DD was born).

I was a bit worried too TBH and I think I asked my ob at the time. Can't remember what he said but I've been fine (some of the weight creeped back anyway because I found that I was ravenous when I was expressing so would have a huge bowl of spag bol for breakfast).

But get yourself checked out if you're concerned. Worth putting your mind at rest.