Hi, I have a litter of 6, 5 day old puppies,
Mums milk isn't the best so I'm topping them up with a bottle,
mum will allow them to feed but won't do anything else, I'm stimulating them to go to the toilet, but I haven't seen them poo in around a day, the last poo was firm,
Puppies are thriving and content, bellies are soft, could they be constipated?
Thanks in advance

I would not be topping them up with a bottle.  At 5 days old, you really haven't given mum's milk supply a chance to respond to demand.  The more milk the pups demand from her, the more she will produce.  If you supplement, they are not going to demand as much from her and you create an ever deteriorating vicious circle. 

Yes, very occasionally supplementation is necessary but FAR TOO MANY people I see on here supplementing instantly. 

Mum's milk is best.  It is WAY better than formula.  It contains everything the puppies need.  Everything about human breast milk versus human formula for human babies applies equally to puppies.  Do not supplement unless it's an emergency.  Supplementing mum's food with fenugreek helps milk supply.

If they are constipated, it could well be due to the formula...