I have seen that before on deer. Believe it or not I know someone who took a deer with the same bulge in the same area. Well he took it to the DEP officers and they cut the sack that was filled with a yellowish liquid. Inside floating freely was a broad head. The officers explained that it is common for a deer to be shot ( bullet or arrow ) and not be fatally wounded. With the bullet or broadhead stuck inside the deers body itwill form a protective sack around the object and very slowly push the object through the skin, it's natures way of protecting them from injury's.