I'm not very experienced myself but I'd say your lights to close may be burning it slightly, looks a little young for deficiencies coz I don't think you feed them nutes till flowering really (bear in mind I've only grown autoflower strains) unless your giving them nutes now and you over did it a bit or it could be over water or underwatering that is most. Likely the problem. Mine did the same but they turned out fine a bit later on