I agree with all the advice below.
It's common. It can be serious, and if you feel like you have a fever, you need antibiotics. Does your breast have any red streaks on it? That is another sign of infection.
In addition to seeing the doctor, nurse on that side as much as possible today (use a pump on the other side if it feels full). Sometimes it also helps to put a warm compress on the lump right before you nurse. Then, while you are nursing, massage your breast by starting with your thump clear at your ribcage above the lump. Then slowly run your thumb down towards the lump, putting some pressure the whole way. When you get to the lump, put pressure right behind it, hold it there for a few seconds, then keep going past the lump. Then start at your ribcage and do it again.
To answer your question about your supply - your mother is right on this one. It's not because your supply is drying up. It is a plugged duct.