Help please!?!

My almost 11 month old has recently stop drinking all his formula won't finish bottles etc. He eats solids fine. We were given the okay mix a little bit of milk with his formula. Tonight I went to do so I made his bottle (we use Similac pro sensitive ready to feed) and I mixed just an ounce- two with his formula and did so while I was making dinner. After dinner and bath (not more than an hour) the bottle was curdled? Any ideas why? I've never seen anything curdle milk so fast. The milk was good I drank a cup when I made his bottle.