rebelroy may be on to something here. Ganja loves Mg and S. I would suggest a dose of Epsom Salt. But in the meantime, foliar spray with some Cal/Mg if you have some... Plus, keep your watering water Phed to 6.25 and always check your plant run off Ph. It needs to be anywhere from 6.2 to 6.5 also...
Any red on the leaf stems, is more than likely the first signs of Mg deficiency. Although, some strains may do this on there own... Can't wait to see the pics. That would help us out a lot...