Pregnant Mare?

Question
We just bought a 13 y.o. solid paint-bred mare. She has had at least three foals that we know of. The previous owner said that she was "exposed" to a stallion and could possibly be pregnant. Doing my best math according to what he told us, I thought that she could have her foal in September, early October if she was pregnant. I had a vet do a blood test in early July that came back negative. However, in the last month she has become increasingly moody and very intollerent of the 4-year old mare that shares her pasture and other food sources. She seems to be getting bigger and does not tolerate her saddle, even less with a rider. She just doesn't seem to be herself, but otherwise in good health, no worms, eating normal, etc. My question is, how common are false-negative pregnancy tests? Are there any other health problems that would cause these types of changes?

Answer
Dear Morgan,
           False pregnancy tests in horses can happen just like in humans, About %20 of the time. Just keep an eye on her, if her udders begin to swell she is definatly pregnant. The signs you are telling me means that she is pregnant or having a false pregnancy. Get a differant vet out to check her, but instead of a blood test do a urine test, or both.
michelyn