I had the same problems with Venlafaxine which is related (both are SNRIs). GI problems are not uncommon with these. Many anti depressants, particularly those that have an effect on serotonin levels can have GI effects because the gut also uses Serotonin. However in my experience I didn't get as severe an effect with SSRIs like Citalopram and none at all with Mirtazipine (although it can increase appetite which isn't always desirable either).