A lot of QH's won't get very soft around the tail and hip area just because they are a breed with really muscly hind ends. Lol.
I agree your mare looks pregnant. Have you tried sitting out with her for awhile with your hand on her belly? Most foals are active and you could try to feel it that way. I have a mare who was not suppose to be pregnant, but I thought she was (this was before I learned how to ultrasound) and I sat in her stall for an hour with my hand on her belly.....sure enough, I felt the foal kick nice and hard. That was 2 months before birth.
Also, if she is testing in the 300 range of calcium, then I would say that's a pretty safe bet that she's preg. Make sure she is getting her minerals and a good Mare and Foal feed. Good luck!
A good horse is worth more than riches. ~ Spanish Proverb