Great advice in here. That used to happen to me in when my post workout shake was 80-100g of fast digesting carbs. I would suggest switching out some of the Karbolyn with a low glycemic carb. You could do 50g of Karbolyn and 50g oats, or have a snack to eat along with your shake after training (whole wheat bagel, rice cakes, etc.)
I've also found that having any solid food helps a lot with the lethargy. Liquid carbs and protein are an easy post workout, but you'll benefit from adding in some lower glycemic carbs. You can add in fats if you like, but I typically avoid them in my post workout meal so they don't slow down digestion and delay muscle protein synthesis - and that may be total broscience too lol. But that's just how I've always done it