How are you feeling?
What you are describing sounds like a blocked milk duct which you may be able to fix before it turns into mastitis. But if you are feeling tired, head achy, sick, like you've been hit by a bus, like you are getting the flu (or start feeling like this in the next few hours) then you could be pretty sure that mastitis has already set in.
But in the mean time do what Miss H has suggested, get that leftie in the shower or under a hot flannel before a feed, massage the spot during the feed and see if you can work that blockage out. Like Miss H I've had several blockages but only one couldn't be cleared and ended up as mastitis.

Hope you can work it out soon.