Have any moms noticed this? I've never heard of it...?

I have never had that before a period, only directly after. That is kind of unusual. I've heard of all kinds of thing, but not this. Maybe it's just that you were going to get a period and didn't because you're going to have a baby instead. Logical enough explanation for me. I guess just ask your gyno about it. Sorry I couldn't really help. Congrats on the pregnancy!!