It's actually really bad sometimes to get the cats spayed while in heat. Not only because they bleed more, and there are several cases of kitties coming out just fine from it, but my friend, Amber, had to spay her cat while in heat because of the same reason, she kept going in every time they rescheduled.

She got really bad repurcussions from that, like she used to be a VERY sweet, lovable girl, and very docile and healthy, and as soon as she was spayed she suddenly undergroomed big areas and over groomed others to the point of bleeding, she's become violent with anyone other than immediate family (she used to be a HUGE people lover) and can't weigh more than like... five pounds or so, which is SKINNY for a cat her size, which is just a tiny bit smaller than Clover.

It messes with their hormones a lot when you spay them while in heat, which is why most vets refuse to do it.

BTW: Hope you can get through it; Clover went into heat before we could spay her (fortunately only once) and while she didn't yowl, she'd whine like a dying mule and pace throughout the entire house and wave her ass in your face while you were sleeping. I know how it is!
Hope your ordeal is over with fast and soon!