Thanks everyone for your helpful replies! I don't think it is a blocked duct, I had one last week so I know what they feel like and I definitely don't have anything like that now. I think it probably was from poor attachment, when I was feeding him it didn't feel right, but because I was sitting in a different position to usual I couldn't seem to get it right. The spot is still there and it still stings a bit, but I will keep putting heat on it and see how we go. If it gets any worse I will definitely get it checked out.

Thanks again everyone!