Periodic problem, stuff I forgot to mention (6)
I believe the answer is:
cramps
'periodic problem' is the definition.
(menstrual cramps caused by periods)
'stuff i forgot to mention' is the wordplay.
'stuff' becomes 'cram' (synonyms).
'i forgot to mention' becomes 'ps' (as in a PS in a letter).
'cram'+'ps'='CRAMPS'
(Other definitions for cramps that I've seen before include "Muscular seizures, spasms" , "Muscle fatigue" , "Squeezes" , "Painful tightenings of muscles" , "Muscular spasms and pains" .)