In all likelihood there is no cause for alarm, as most breast lumps are found to be benign.
However, all breast lumps should be evaluated by a doctor.
The lump could be a fibroadenoma,
which is rubbery and round, and slightly tender,
Or it could be a cyst
which is caused by natural hormonal fluctuations and is also tender to touch.
If the skin above it is red and warm,
this sounds like there may be some infectious process going on.
In that case, it may help to apply a warm compress until she can be seen by a doctor.
Then, the doctor can evaluate the lump
and prescribe a plan of treatment.
Here is a link regarding breast lumps and the various types and treatments:
But the most important thing to remember is that ALL breast lumps should be evaluated by a physician and in a timely fashion.
As I said, it is unlikely that the lump is cause for alarm,
but if it is a cancerous or malignant lump of some kind,
then the sooner it is evaluated and treated, the better.
I hope this is helpful to you. Please feel free to ask additional question if you have one.