It's unusual for the vet to do a spay while the dog is actually on heat. Mostly it is performed in between heat cycles. So the reason she was spayed at such an unusual time...? Did she have an infection or some trouble to start with? Or was it an unwanted pregnancy that was aborted by spaying?
If so, the problems could stem from the initial reason for spaying.
If not, what Margot suggested is a good idea. Keep a good watch on her temperature. If I were you I would take it at least twice a day, in the morning, and in the afternoon. Above 102.5....103, speak to the vet because of possible infection.
But maybe this blood loss is quite normal and a result of the surgery. It may well stop in a day or so. I would definitely discuss this with your vet though. A telephone call will not cost anything, and the vet will not mind.
Since she was in Season when she was spay, you may be seeing blood from the surgery and not necessarily coming from the urine. The clump blood could just be a residual blood clot from the surgery. Do you know how to take a dog's temp with a rectal thermometer? Any sign of a temp over 103 can be the signs of an infection.
If I were you, doesn't hurt to give the Vet a call and run it by him and see what he says.