When you say her heat ended 11/25/06 was this when the bleeding stopped?
The only possibilities we have here is that she had another silent heat due to irregular cycles and your two dogs did get together or she had a visitor into the back yard when she went out to potty.
Yes a dog can produce sperm as early as 4 months old though it is usually between the ages of 5-6 months and at four months they aren't very coordinated to figure out how to breed though some have.
Or, you were scammed. The problem is with this sort of scam is you will never be able to prove that your dogs never got together or she didn't have an unwanted visitor in the yard. I would hope that the SPCA wouldn't be so ignorant to do such a thing to clients unless they were out to try and prove a point.
If you are 100% sure there is no way she could have been pregnant then I would demand a pregnancy test today through another veterinarian as she should have some pregnancy hormones (Relaxin) still floating around. This will prove to you whether she was pregnant or not during the time that she was spayed and if she wasn't then you have a suit on your hands.