It sounds like a floater
@CrankyKat. Does it move around? I have a 'weird black stringy friend' too - this appeared very suddenly as a large mass of black smudginess and threads, after flashing lights. A trip to the eye clinic and it was diagnosed as a floater, which is caused by the vitreous shrinking. "A bit of **** that got dislodged" is a good description! Mine wasn't caused by treatment though but by very short-sightedness, and
@Jaylee may be able to give you more info. The stringy friend diminished over time and is now just a curly thread that I can see in certain lights. Fingers crossed yours will disappear and all will be well.