You r underweight only and not at the age of Marriage. So dont bother until it happens . Just avoid all junk foods, aerated drinks , sweets n chocolates, ghee stuffs n baked items. Eat only healthy food n drink plenty of water . This may be result in better in future . Dont go for treatments now itself . you can better check it after your marriage. PCOD dont have permanent solution. It may occur again n again once if cycle rotates and also if Harmones imbalances.
So my suggestion to u is maintain above said diet n quit treatment n consider treatment only after marriage if comes necessity.
Then to work out to period issue eat daily dates with honey -LIon dates syrup is also a best solution for this. Start working out before marriage. Otherwise it will affect your pregnancy later.
Foods you can intake is : Any wheat based item like chappathi, roti ,corn flakes or oats. Rice with vegetables stuffed. Fruits like apple, orange, pomegranate , cantaloupe ,watermelon except grapes, sapota, pine apple etc. Avoid Meat of Goat. You can eat chicken n Fish. Avoid Oil items, snacks, sugary items, fryums n fast food. Drink plenty of water. No juices other than lemon only raw fruits .
dont be tensed. Be relaxed n work out all these. coz tensing may cause stress n change ur harmone levels too much.
Take care dear