this was taken from Mark Mandics breeding page ( http://www.ballpython.ca/gallery/breeding.html )

"Tail Suck!

When they ovulate, their tail looks like all the fat is being sucked out of it. It looks very scrunched. Sometimes they will have a scrunched tail prior to ovulation without the body swell. This is just part of the building process."

IMO thats what it looks like you got there, the "tail suck" comes after ovulation and before the pre-lay shed