Sore nipples breastfeeding 9 month old

The past few days my nipples have been somewhat tender, and my upper back is sore/tingly.

I’m breastfeeding my 9 month old.

I don’t have reason to believe I’d be pregnant, but I suppose there’s always a slim chance. What do you think?

I have a few theories on what could cause this, but I have no idea if they make sense:

• My son is larger now so he has a stronger latch.

• He’s starting to teeth again

• He’s heavier now (over 20lbs) hence the back pain (I have a bad back)

• I bought a new nursing bra….the back pain is upper back around where a bra would go. Also could a new bra cause nipple pain?