Give her a sippy of milk, instead.
I know the feeling.
When I was pregnant once, my daughter was still nursing.
It hurts.
TOTALLY sensitive nipples.
Egad.
Well your daughter is 18 months old. She I assume, is on whole milk by now???
Or you just need to distract her, when she goes for your boobs.
Kids that age can get distracted easily.
Or just don't sit down or lie down. Tell her "Mommy is busy.... in a minute..." then MAKE yourself busy. And she will get distracted.
My daughter self-weaned at about 2.5 years old.
That is what I would do. Distract her sometimes.
Then again, the sensitive nipples, does go away after a time.
If you can put up with that or not. Its up to you.