I don't know if the drugs the mother is on could cause the milk souring.
The most common reason though for what you describe is the milk fat
being broken down by the mother's lipase to free fatty acids.  Some
mothers just seem to have a very large or active amount of this enzyme.
If this is the cause, the milk is still good even if it tastes bad and
smells bad.  It is extremely unlikely for breastmilk to spoil so quickly
due to bacterial contamination.

The amount of drug in the milk probably has little effect on the milk,
but I can't be sure.

Jack Newman, MD, FRCPC