Yeah, he probably isnt the right one to try and fool. He likely knows her species anatomy and birthing process just as well as she does, if not better. Also, its been like 2 hours tops. She would have had to be impregnated and managed to grow that in that time. Idc even if you are part snake, tissue does not stretch that fast, let alone painlessly. It needs time, and resources (for making new tissue, blood vessels, nerves, etc) to stretch. That mass does not come from nothing.