I don't know if it is normal, but I had the same issues nursing my toddler. I started noticing teeth marks on my nipples (not painful) when she was a little less than three. Then when I got pregnant it was unbearable (and her latch seemed to get lazier). It almost felt like she was forgetting how to nurse. I did ask her to reposition herself sometimes, and would try to explain that what she was doing was bothering me, but it wasn't a great situation - she didn't seem to know what she was doing wrong and we would both get upset. We finally weaned, due to decreasing interest from her (and maybe less milk?) and excruciating pain for me. I also thought it might have to do with my breasts being engorged - so harder to latch on to.