If only from left breast and accepting feeds from right breast then more chances of to get some local pathology in left nipple either cracked nipple or soreness or retracted nipple. Examine your nipples if you find any local pathology. Apply last milk part of breast milk ( hind milk ) over nipples which wil help to prevent from soreness. If retracted nipple then you may need syringing for retracted nipples. If still symptom persist then review with pediatrician.

There is nothing to worry, you should check your left nipple, to see any infection, soreness etc, Side  by side you have to evaluate your baby's painful conditions, like any muscle pull of any limb, any ear pain on gentle press on tragus of ear