As far as your body tissues are concerned, your body weight will more or less be the same. The mass of the tissues hasn't changed, and therefore the weight of the tissues won't change.
What you are probably experiencing is the extra mass that is a result of blood flowing into your manhood. The additional volume of blood .If you "weigh" a lot in a flaccid state, are you also a "shower" and not a "grower"? I suspect that your penile tissue retains a lot of its blood.