It does sound like mastitis. Try warm compresses on it, and you can try that while you're pumping that side. I've also heard cabbage leafs (for a short amount of time because it's soo effective) can help draw out the clog in the duct. If it continues don't wait, go see your doctor because it will get much worse.
Even if uncomfortable completely empty your breast. Massage and heat will help too. If you notice your breast becomes hot to the touch and red you should be seen my a Dr because it might require antibiotics
There's a really good cream u could use called lanolin and the other good thing is its safe to leave on while baby feeds no need to wipe it off.
I had mastitis twice within a year.
There was engorgement and a stinging pain but also a reddish line and increasing fever. I went to see a doctor and was told once the reddish streak appears and fever starts, there is no way around antibiotics. As much as I dislike them and worried about baby, the pain and redness disappeared within 6 hours. I had to feed from the unaffected breast for a day and pump the other.
Hopefully you`ll catch it on time with cabbage (I hear it works) and yes, lanolin is mom`s best friend! Heals small injuries quickly so mastitis doesn`t even have a chance. Hang in there!
I agree w/ all the above advice. Hope you are feeling better now :)