My penis is visibly bigger, although still flaccid, if i've been holding my pee for say 15-20 minutes. is this normal , and if so, why ?

Yes: This is a normal physiological response. Because the nerves that innervate the bladder and the penis are the same, when the bladder is full it causes a reflex stimulation that promotes or facilitates erection without any sexual arousal. This is why it is common to have an erection in the morning sometimes when our bladders our the fullest from overnight sleep.