Ay, normal yan sa breastfeeding, mama! Sa 6 months pa lang, marami pang possibilities, usually it's either a clogged duct (pag hindi completely emptied ang breast, nag-build up), poor latch, or sometimes thrush din. A few of us mum friends had this, at ang usual suspect is talaga yung clogged duct, kaya importante mag-feed frequently from that side at mag-massage habang nag-BF para ma-help ang milk flow.
The thing is, kailangan mo rin i-check kung may fever ka, if the breast is hot to the touch, or may redness, yun na signs ng mastitis, which needs immediate attention. If walang ganyan at it's just tender, try warm compresses before feeding at cold compress after, para ma-ease ang soreness.
But honestly, since it's been bothering you, mas better makonsulta mo na ang doctor or lactation consultant mo para ma-rule out anything serious at ma-address properly, better safe, diba?