Nipples bleeding and sore - help?

I stopped breastfeeding my 13 month old for two weeks because I’ve been ill and didn’t have the energy/ability to. Now I’m back to nursing at night again. Two things I’ve noticed: she’s getting frustrated because my milk supply has drastically decreased, and it HURTS. Only two days in and my nipples are cracked and bloody. They’re extremely sore. I haven’t felt like this since my first child was having latch issues, I never had any pain with this baby even when she was a newborn. Why does it hurt so bad now? How do I stop it?