Plan b?

I had the Mirena inserted 2 days ago and the doctor said it is effective immediately. So my boyfriend and I had sex without a condom today. Now he's worried because I started spotting two weeks ago for a few days. It was never on a pad, just when I wiped. We have a 2 month old daughter and I'm breastfeeding. He thinks I should get plan b just to be extra safe because we don't want another child. The question for the poll is if I should take plan b or if I'm safe. And then a question for comments, was my spotting a period?