Could it be some kind of insect?

Nah, you'd see holes and that. I can't think of any insect that makes your plant shiny pal, maybe I'm wrong, but I've yet to c anything like that that isn't early-mid N tox pal that's all. Your leafs are very shiny and green are the crispy, that looks it? The way it's drying out and all wrinkle. Just N tox. Cut back on it, give a few water feeds if using soil if coco flush, increase micros lower grow,bloom for the next 2 wks increase cal mag slightly. Then back to normal but get back there slowly. Add extra micro to match at least 2 nutes ml for ml when you restart. Good luck happy growing.