That's just an erection. It happens when tissues in the penis called the corpus spongiosum penis and corpora cavernosa penis fill with blood, causing the penis to become longer, thicker, harder and to stand upright. This can happen if he is touched sexually, (like you mentioned with your mouth) or even if he is not touched – for example, if he sees a naked person or thinks about having sex with someone else.
So you see Kristine, the penis in your mouth isn't actually growing, you are just causing the man to be aroused. That's all. Not a miracle, not healing and no special powers. Just biology.