Mastitis?

The night before last my baby slept longer than usual and I woke up with a sore breast. I fed her on it and it felt better. Throughout the day it felt a little tender before it was time for her to eat, but once she ate it felt fine. (She only eats on one side at a time, so each breast is emptied typically every 5-6 hours. But overnight I make sure she eats a little on both sides.) Yesterday afternoon I got a headache. This morning I woke up with a headache and a tender red breast with a swollen lump. I have a clogged duct and I’ve been feeding on that side often. Otherwise I feel fine. It’s not very painful, just tender and I have no pain with breastfeeding. Is this the start of mastitis or just a clog? Any tips to relieve it?