First, Congradulations on breastfeeding past 12 mths!!! You are now doing what is considered 'extended breastfeeding'! :D Welcome to the club; its very small :D (smiles while joking about 'club').
Secondly, like the others have said, it shouldn't really affect breastfeeding.
I started my period at 18 mths, 15 mths, and currently still awaiting 'Aunt Flo' to return... my baby is 15 mths now :) I'm hoping to go another few months... Although, you can still get pregnant without any bleeding...
Oh, and if you need any support while practicing 'extended breastfeeding', you can email me :D I've been breastfeeding for over 8 yrs now... continuously... :D