There is no clear link between shorter cycles and low egg quality. The advice you have received from your GP is good and you should carry on trying. If you wish to have some investigations  I would advise a day 21 progesterone test ( 7 days after ovulation) to assess if you have any luteal phase insufficiency and an AMH test to assess your ovarian reserve.  If your luteal phase progesterone is low then you can be prescribed progesterone support.