yeah it could be heat stress. my guess is salt build up. if the leaves curl like that and become crunchy it really could be salt build up, which locks the roots and they cant get nutrients. so stop fertilizing for a day or two, and water the hell out of the plant. like bring the plant to the bath tub and start watering and letting the water drain from the pot its in. thats the most important part!! it doesnt look like its too bad yet, so get it when its early. and if your temp is like 95 degrees, just get a fan and dont pay attention to what i just said lol.. good luck man!!