It is recommended that babies be exclusively breastfed for at least 6 months and supplemented with breast milk till the age of 1 year.
Since she is 8 months, if nursing is a pain for both you and her, perhaps you could switch to feeding her expressed breast milk.
If not, maybe she could be uncomfortable in the position. You can try switching positions so the both of you are relaxed.
She could also be having a growth spurt.
http://www.breastfeeding-problems.com/baby-fusses-at-breast.html
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