She's really on her period, in which case I would definitely prefer NOT to do it (my experience has not been positive. It might be okay in the dark, but really nasty when you take a shower afterward and see all that red stuff running from your organ down to your legs).
I've always found it fine. Drop one or two bath towels on the bed, possibly do it in the bathroom, and everything's okay. If you shrink at the sight of blood it's probably not for you but if you laughed through the first part of Saving Private Ryan by-all-means don't shy away. I wouldn't recommend the tongue action though
One really bad experience in particular comes to mind though. The chick didn't know it was coming or wasn't prepared, I don't know, so I didn't know it was that time of the month. I slipped off the panties and mounted - as soon as she spread it was like a friggin fire hose. It was like a giant, red, liter of nasty that left her body like a blue balls cum shot. My crotch and legs were covered and the sheets were ruined. I got a little angry then just laughed my ass off as all I could think about was Your Friend's Neighbors.
All told, I screwed her an hour later