Try magnesium malate, Mg is often needed to help the body transition from a standard diet to a more ketogenic one. Also take a probiotic, that helps balance your gut flora, an imbalance of which is a major cause of constipation.
A combo of the recs from above have worked for me - more fat, more water, more greens, and Natural Calm morning and night. Probiotics are helpful as well. If you have problems with digestion, digestive enzymes can work wonders.
I would try a 1/4 teaspoon per day before going to sleep of Epsom Salt, the edible kind, dissolved in a glass of water. That should make you regular, and that's what you call "constipation." If not, then you need alternative methods that may require long-term treatment, in case your gut flora is damaged or your glucose intake is insufficient or your colon walls have no moisture.