Thanks for your question.
This is a fairly common situation, unfortunately, so there are data to support an answer to your question. Cocaine is fat-soluble, meaning it can accumulate in fat. Since breast milk is largely made of fat that is intended for the baby, cocaine can remain at high levels in the breast milk.
As a pediatrician, I routinely recommend that if a mom is using cocaine, or used it recently in the past, I would avoid breastfeeding altogether. The risk is simply too great to the young baby, and in my experience most cocaine use is repeated. Rather than worrying about each use of cocaine, I recommend the use of a cow's milk-based formula like Enfamil or otherwise.
I hope this helps, and I wish you and your baby well.