Help! My breast hurts so bad

Hi! I've been breastfeeding for 15 months and never had any problem, today I woke up and my left breast hurts and it's kinda hard, it feels kinda hot when I touch it too.

I tried breastfeeding my baby because I thought maybe my boob was too full or something but it still hurts and it's so painful I don't know what to do!

Does anyone know what this could be? The sooner I can go to the doctor is on thursday but is so painful that I don't know if I should go the er or something. Help please I'm scared!