What is this inside Arla Bob skimmed milk????

I heated up milk for my boy and as he was drinking the bottle got clogged and it looked like a white string that I first assumed was from heating up too much and they were the bits that get stuck at the pan. I removed it and it happened again which I then decided to pour the milk into a sieve, and washed the milk away with water and the remainder looked disgusting like a phlegm texture, it wouldn’t separate and it got a translucent colour (picture 1). I then heated up a different sealed milk, poured it on the sieve there was nothing so its not from heating the milk. But you can see the pot has bits stuck to it from heating (picture 2) so I scraped it off with a spoon then poured that on the sieve and washed it like the 1st method but the colour was white and separated like I expected from a usual normal milk (picture 3). So what was the 1st picture? If the milk goes bad, you will still get the same result as the picture 3 not 1…