Easy answer. You stirred it too much. Too vigorous stirring when you mix the yogurt start with the milk will result in stringy yogurt that is impossible to eat (disgusting, in fact).
Instead, mix the start gently with a small amount of milk, gently breaking up the blobs with a whisk or fork, then add it to the rest of the milk and stir gently just a little. Don't worry if the yogurt start still floats around in small blobs. That should fix the problem.
I make my yogurt in pint jars and "check" them by turning them upside down to see if they have turned yet, sometimes checking too many times. The result can be really nasty, stringy stuff.
Hope that helps,
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