I'm sure with Milo I used to get them around the same time every day, I can't remember exactly when, but I think it was around late afternoon/early evening I'd suddenly feel them.. it's strange isn't it!
I always put it down to getting a spurt of energy and doing too much in the afternoon, but who knows, it may not be connected at all!
I'm a fat lot of good aren't I!
But if it helps you feel less weird, I'm sure I noticed that too when I was pregnant.
And blimey, can't believe you're 31 weeks already! Exciting times ahead!