I experienced that VERY issue for the past couple of days!
It's your milk, it's gone bad, try making another cup of tea with that same milk, you should see this again, then AGAIN with a fresh carton, it shouldn't happen!
The possible problem, may be that your milk hasn't been in a cold enough fridge. My fridge had that thing that makes it cold got blown, thus my food spoiled quite quickly.
Happened to two or three large and small cartons.
It's now fixed, and my milk's just fine! :)