If it isn't from the 120F weather you have been having, then it would be very very very slight fert burns to me. You might have gotten a drip or two of solution on your leaves as you were watering, and it wasn't a foliar spray, OR there was just a smidgity-smidgen-smidge too much ferts used, and your baby didn't like all that juice. . . It's no biggie, you 99.9% likely won't even lose a single leaf because it does not look severe enough, whatsoever.
My main suspicion, the other thing is, with 120F heat, that is simply not human. . . or plant for that matter, and I believe strongly that the weather is to blame. One possibility is it has caused simple over-transpiration (Too much moisture loss at once - consider it the plant form of sweating). Plants like marijuana do not generally enjoy temperatures beyond 105 (105 is the most extreme case), BUT it also all depends on pheonotype. Some strains are grown in extremely arid, hot climates, and thus are much more likely to thrive in high temperatures. Others thrive in cooler or wetter, or higher or lower latitudes, etc. . . I mean imagine if an indoor grow room reached 120F. . . the effect on your plants would probably be deadly!
