He could pf had a bowel prolapse? This is when the walls of the recum pop put of the anus,this could be due to Arnold straining when he's trying for a poo,is it possible he could be constipated? What foods do you feed him on? Have you checked his cage for poos? Maybe if there's not many there he could be abit constipated? Is he well other than this lump you've found?
Giving him a little fresh fruit and veg can help with his constipation (if it is that)
Or it could be his man bits,they do stick out quite far.can you tell us anything else about them? Xx