I'm thinking breathing too. But what are you eating on these days and how many calories, and what are you eating around your workout time? EC can easily make you nauseous too. I used to not have an issue with stims, now I do. So just because you never had, hell, if you keep at it, sooner or later it may catch up to you. Remember its a controlled malfunction of the endocrine system and don't think it's anything else! And as someone said a while back on this board. Nothing gives you energy. It just borrows it from other sources. There is a toll to pay. Sooner or later.
When doing your fullbody routine do you do shoulders\upperbody right after legs?
I don't know how to say it in English but you may be experiencing lack of oxygen to the brain because you're not in enough cardiovascular shape for the heart to be pumping to both your legs and upperbody without rest.
Usually when i do fullbody i do legs with 1m30s rest intervals between sets then i rest about 3min before doing upper body exercises.