Plan b.. While breastfeeding..!?

Okay so long story short, I need to take plan b... I'm 8 months post partum, I have already gotten my first period... I basically exclusivly breastfeed...AND

I'm worried about a reduction in breast milk.. How do I prevent this?

I'm also worried about the chemicals getting into baby..

I breast feed on demand so it's not like going hours with out BF is realistic for me..

I've taken plan b before.. When I was about 2 months PP.. And I could've sworn my milk reduced... But I'm not 100% sure it was because of the plan b.. I wasn't even a lot due to depression around that time so maybe it was the lack of food too..

Help :(