wow 5 oz

thats amazing !!
has something changed in your routine that could explain the drop ? are you expressing at the same time as you was? are you as relaxed as you was? are you comfortable and un hassled ? was you doing anything to help you before like loking at a photo, drinking something hot, spending time massaging before starting ?
im having to top up with formula at the moment every now and again he gets 2 oz but i try express 1oz so that i can make it half and half
could you do the same ? keep expressing and trying to get your supply back up and just add 2oz of formula to what your expressing. hopefully your milk supply will increase again and you get cut out the formula
or could you try expressing at other times ? yu can keep adding to your collected breast milk through out the day,