It sounds like it was clabber - naturally thickened milk, due to the lactic bacteria that are in raw milk.
Had you treated your cow with antibiotics? That may be why the rennet didn't work. Or the pH of the milk was off from the mastitis.
I don't think I would have kept it, either, knowing that it could have been from the mastitis. Once she's healthy again, though, you can try deliberately clabbering some milk. This
is a really good thread on making clabber from the Keeping a Family Cow forum. The first reply by Lannie gives an excellent description of how to get a good batch of clabber going.
I have made a parmesan-like cheese by innoculating a kettle of milk with good clabber at the rate of 1 cup clabber to a gallon of milk. I add it after warming the milk to about 100 degrees, then wrap it in a blanket to keep it warm overnight. It is usually a near-clean break coagulated by that point, so I cut the curd and gently warm and stir it until the curd shrinks and the whey is fairly clear. Then I drain it and press it, working up from 20 lbs to 60 lbs in successive pressings, with the last being about 12 hours or so. I then brine it for 12 hours and dry it in the cabinet for a couple of days before putting it in the cheese cave. In about 60-90 days it has good parmesan flavor and isn't as much work, IMHO.