The thing your ex-girlfriend does, after you break up with her. (for whatever reason)
In lack of comfort you gave her she is emphasising her efforts in finding a new person or at least getting the comfort of being in someones arms for the night.
While you end up alone noticing that girls aren't that easy as they used to seem, when you were in a relationship, she gets lika a million dicks within a short period of time.
A fuck spree can also be an "insane fuck spree", which is the same but with more guys your ex. of a fuck spree is insane depends on your ex's appearence, age and sexual attitude.
Expect the fuck spree to be insane if she is hot.
a: Damn, whats up with your ex? SInce you broke up with her i see her everey weekend disappearing with another guy from xy-club
b: yes, she is on a fuck spree... man
a: dude, i thing she is already on an insane fuck spree.