This sounds like over fertilization to me. The reddish/purple stems are usually an indicator of too much phosphorus... and it wouldn't surprise me if that means there's too much of a few other things.

HOWEVER! No one reading this post has any clue as to what/how much you are feeding! a very important thing to know regarding questions about feeding!
There's also really no way to tell you how to fix the problem without knowing what medium you are using (soil, hydro, etc.).

If you are using soil, would suggest using straight water for your next few waterings and see if things get any better. always good to keep that PH between 6.3-6.8 as well.

Beyond that, Fill out this form:
http://boards.cannabis.com/plant-pro...ing-forms.html
It gives anyone reading this a MUCH better handle on what you may be doing wrong.

Happy growing :jointsmile: