A 32 year old female presents with a 6 month history of intermittent bloating and abdominal pain. She has noticed her stools are much looser (although only opens her bowels once a day). She denies any blood in her stools and finds her abdominal pain generally settles once she has opened her bowels. She further admits to drinking a lot black coffee in last few weeks to keep alert as she is trying to meet a work deadline. She had the Mirena coil inserted 2 years ago as she has always suffered with menorrhagia and she has completed her family.
What is the single most probable diagnosis in this case?