It could be a blocked duct. Not mastitis yet most likely.
First, take it as easy as possible - act as though you are ill and do NOTHING - if you get mastitis you will be laid up for about a week or so so a day or two in bed is worth it to avoid that.
Look up hand expressing technique - this is what I found most helpful. Think about your baby or look at your baby while doing it to help letdown, you can also stimulate letdown by "drawing" (with your finger) lines from the fuller part of your breast to the nipple all around (like sun rays) and a spiral going from the fuller part of your boob to the nipple. For some reason it helps, probably psychological. Do this before you start hand expressing.
If you can find where the actual blockage is, it's best to try and get that out - you can use a blunt, wide toothed comb in the shower to "comb" it towards the nipple, when hand expressing put your thumb on that side, when feeding, have baby's chin on that side (which may involve acrobatics like lying them on a bed and dangling the boob in from above if it's in an awkward place)
You can take ibuprofen to reduce swelling. Also alternate cold/hot compresses. Cold to reduce swelling, hot to stimulate letdown to get the milk out. A pump was totally useless and painful IME.
If you feel flu like symptoms then go to your GP/out of hours immediately as you'll need antibiotics to clear the mastitis ASAP.
Good luck!