It's hard to know - it could be either. I would probably work on the basis that it might be a blocked duct and try to clear it. Warm compress before feeding, feed from that side as often as you can, massaging it gently, cold compress after a feed. Changing feeding positions on that side might help too. If it gets worse maybe try hand expressing in a warm shower, leaning forward into a bath or bowl of warm water and hand expressing or feeding on all fours so that gravity is working with you.

Hopefully it will clear quickly for you