Couple things it could be...
It could be Heat Stress, as RockstarEnergy posted above. This will happen when the plant is in a warmer than usual enviornment and is going thru higher levels of respiration and trying to cool itself. Fall is about here, so if it's heat, you should be in the clear soon enough. Best option would be to spray down with cool water in the mornings around 7-9am before the sun is intense, and then spray down again in the evenings anywhere from 4pm and on. By avoiding 10am-3pm, you are avoiding the most intense parts of the day. If you attempt to spray your plants down, you will burn. The good thing is that the morning spray will help them take that period a lil better, and then when you spray at 4pm or later, keep in mind that 4-5pm are the hottest points of the day. So by spraying then, you'll be cooling the plants down greatly and giving them relief @ temps they normally won't see til 9-10pm n on. They WILL thank you for this misting if you take the time.
If it's not heat stress, then the "cupping" of your leaves could be what's called "praying for magnesium". This is what happens when they are Mag defecient and need a boost of those levels. I am currently using botanicare's Cal-Mag Plus to solve problems like that, but there's plenty of other products on the market, and even dissolving some epsom salts into a glass of water can do the trick at low cost.
It could be both problems working in tandem
Or it could be other more complicated micro defecencies, in which I'd need to see more pics or be there in person to see, so I'd recommend to you to try to give the two methods above a shot and if that doesn't work, crack out some information on all cannabis defecencies and any pictures available of what they actually look like, and do some comparisons to see what else it could be.