Yes, it is probably heat stress. Young plants don't do well close to HID lights. Only move the lights closer once the plant has a good amount of foliage, and is growing fast. The younger the plant the further away the light should be. 2-2.5 ft for seedlings is good, moving closer as the plant grows more leaves.

Check out this link for tips on light resource management.:

http://www.angelfire.com/cantina/fou...r/lightres.htm