As Dave says, the role of tadalafil at this point is simply so that there is enough blood flow that the penis remains in good health. Before long (follow the advice of your doctor or specialist nurse as to when) you will probably want to use a pump for artificial erections, again to keep the penis tissues healthy. The "feeling" of an erection is at least reassurance that the psychological stimuli are intact.
I haven't experienced diarrhoea as a side effect from tadalafil, but since your operation was pretty recent I wonder if it is still a hangover from that. The bowel (lies just behind the prostate) gets pretty knocked about when the surgeons (or their tame robots) are at work, and will be a bit sensitive for a while.