I would say go get checked but also it's ***very important*** that you keep feeding on the sore side - if it's the early stages of mastitis or a clogged duct, not feeding will make it much much worse. So, take some ibuprofen for the pain (it will also deal with any swelling) and feed or gently hand express from the sore side.
Have you any red patches on the breast? I had recurrent mastitis (yuck) and would always get a wedge shaped red patch, feel like I had the flu, and feeding was like having shards of glass rather than milk coming through
so hope you don't have that and are feeling all better already.
You can also try changing your feeding position on that side to better drain the breast, maybe that duct isn't being emptied.
Also, be aware that not every GP is well up on BFing so if you think the advice you're getting is dodgy, double-check it. These guys are amazing, have a look at this checklist:
https://www.breastfeedingnetwork.org.uk/if-breastfeeding-hurts-05-may-2015/