Hi Justine,
It could be a blocked duct. Have a shower and let the warm water run on your breast and 'try' and massage the lump - not too much that it hurts. Have your baby feed with her chin pointing in the direction of the lump, wether it be a normal hold or under the arm hold but try to still feed off that side as much as possible.
If it is mastitis I think you get flu like symptoms and a rash. Don't quote me on that though.
If it doesn't clear up in a few days go and see your GP
HTH,
Bec