Could be your period as that can certainly cause nipple sensitivity and sometimes a variation in supply.
It could be that her latch has changed perhaps as a result of discomfort through teething.
You could also 'express' a tiny bit by hand to start the milk flowing just before you put her on incase she's frustrated with a slow let-down.
Sometimes thrush causes nipple pain and things a bit like blisters. This website has a couple of (scary photos) of nipples with thrush.
http://lactinv.com/tscreen.htmI'm not sure how old your baby is but it really is worth waiting until 6 months if you are breastfeeding before starting solids.
The following organizations recommend that all babies be exclusively breastfed (no cereal, juice or any other foods) for the first 6 months of life (not the first 4-6 months):
World Health Organization
UNICEF
US Department of Health & Human Services
American Academy of Pediatrics
American Academy of Family Physicians
American Dietetic Association
Australian National Health and Medical Research Council
Royal Australian College of General Practitioners
Health Canada
On the kellymom site you can click on the name of each organisation and find out why.
http://www.kellymom.com/nutrition/solids/delay-solids.html