Girls, could you please tell me what it might be? Half my chest hurts,…

Girls, could you please tell me what it might be? Half my chest hurts, on the left side near the armpit, I'm breastfeeding. I give the baby a suckle, there is no pain. The breast is hot. Measured temperature 37. Could this be due to a breast pump? Just pumping it out wasn't very pleasant for the breast.