I think it's a ph issue. If you don't have a meter, get one. If you have meter, make sure it is calibrated. If you haven't fed them ANYTHING and there is nothing in your potting mix, that is your problem!
There is so much misinformation that gets spread about when to feed. If you are in a inert medium, like coco, promix, rockwool, anything without nutrients-you need to add fertilizer almost immediately. My first grow I listened to peoples advice on forums and didn't feed them for almost two weeks and that is exactly what happens. It starts to cannabilize itself. So if you haven't fed it anything, do so. Start low and slow.
If you don't have a ec/tds meter, GET ONE! If you can't then start at 1/8strength what the brand recommends for the first couple days, then increase to 1/4 strength by the second week. If they start responding well then you can try to push them further. You won't be able to accurately judge you feedings without the meter, but many people grow without one. I don't.