SOunds like it could be a blocked milk duct to me, thats how my mastitus started when i got it, within a few hours i had a large lightning bolt shaped red lines across my boob and was in agony. Went to the docs and a quick course of meds and it was gone. Infact the lines went within a few hours and the pain within a few days!!! but i couldnt move my arms it was so painful! hope it isnt mastitus and you can fix it yourself!! maybe make a doc appointment to be safe!!
hugs to you.