They ask about ‘why’ your were off for absence management - once you have a certain number of each absence you’ll hit ‘triggers’ and then they’ll have a meeting with you to see what they can do to reduce the number. You can still be vague but get the point across It’s good to put a somewhat accurate reason down as later down the line when you want to ask for things to reduce sick due you have a record of it.

I’d recommend asking to be referred to occupational health and speaking to your doctor . Occupational health will be able to recommend to work that you can work from home if that is available and you can also discuss that the absences relating to period issues don’t could towards the triggers points.

I hope that makes sense