Did plan B work?

I took a Plan B on the 15th right after unprotected sex and I got a period the 25th. Mind you I hadn’t gotten a period ever since I gave birth 5 months ago due to breastfeeding/pumping. It was very heavy the first two days. Also Now my milk supply is almost dried up. Does this mean the plan B worked? Should I take a test to make sure? How likely is a test to give a false negative?