At 13 months continuing breastfeeding is not all that important. The important thing is to treat your mastitis. This is much easier to do if you are not breastfeeding.
Since you're breast feeding it's likely you've got a clogged milk duct (for more information: http://www.**********.com/0_plugged-milk-ducts_8494.bc).
If you're uncertain, the best thing you could do at the moment is to see if the lump changes in size and texture as time pass; keep track on how its properties are connected to your period and hormone levels in your body. If the lump hasn't disappeared, you should contact your doctor. :)
Most breast lumps are benign. :)