Best birth control while breastfeeding

My SO doesn’t like to pull out and we have a 5 month old. We had sex last month and yesterday he came in me. I plan on taking a plan b tomorrow morning once I get one.

What’s the best birth control ? My OB after my pp appointment gave me some and knew I was breastfeeding but the back of the box says it can be traced in my milk.

It’s called Errin