But earlier in the thread he said he's using a 600w light, more than enough light for such a small tent. In fact, in a 2x2 tent I'd think OP would be more likely to have problems with heat and/or excess light than anything else.
The single bladed leaf is almost always a sign of re-vegging in my experience, used to happen to me all the time outdoors until I started seedlings outdoors in the natural light cycle as opposed to going from 18/6 to the outdoors. Going from 18/6 down to 14/10 is enough to trigger flower for a couple of weeks until the plant realizes the light cycles are in fact getting longer and not shorter.
I only bring all of that up because the issue here is likely not a light leak but possibly something with the OP's timer.
OP, have you checked your timer? Is it in running order? Does it have a notch up in your dark period section of the timer?
I've had the pleasure of making this fuck up before. I bumped a fan into one of my light's timers once and I didn't catch it for a few weeks, resulted in the lights coming on for 15 minutes in the middle of my 12 hours of darkness.
And try using less nitrogen in your next grow, the dark shade of green on the leaves, burnt leaf tips, and downward curling leaves are all signs of excess nitrogen. Definitely not something you want in flower.
Live and learn my friend.