My very first appointment EVER I totally started right at the end. Everything was fine, he even went down on me, and I got on top, came, and when I pulled off......presto! He was incredibly gracious and saw me again anyway. And I have seen two people while I was ragging since then, one used it as an opportunity to break out the strapon and the other just kinda sighed on the phone and said "I've been married for 30 years" and he didn't care at all lol.
And you men with the "you should know your own body", well it isn't always like clockwork. Can you tell you are premenstrual? Usually. But not always. Sometimes it is a total surprise.
Especially if you spend a LOT of time around other women who don't spend time with each other. Women match cycles with the women they spend time with but if you are being matched to two different cycles it can play silly buggers with your hormones.
I've never dated a guy who cared, and I get so sensitive it is definitely one of my favorite times to get messy.