My bitch is due to whelp on Monday.  She started licking herself yesterday and is now losing 'brownish discharge' - she is a little restless so I'm anticipating puppies within 24 hours or so. 
I know this isn't normal but should I be worried?

I have never had a coloured discharge so I can't comment on that but I'm sure someone else will be along soon. If you are concerned you could call your vet.

Is your due date based on ovulation or mating?
I had one whelp 57 days and one 58 days from Ovulation.

Do you mean that you're worried that she will whelp too early?  If that's you're concern then don't worry. 

My breed only usually goes to day 56.  Perfectly viable pups, as the lungs are developed. 

As long as the discharge isn't smelly, if it is then a vet visit is imperative.

Discharge doesn't smell - I'm not that worried about being early as all prepared.  She's very quiet now so hoping all will settle down for another day or so at least