Found a lump under my 3month old daughter's nipple. is this anything to worry about?

Depends: Sometimes this is just maternal hormones that passed during pregnancy & trigger some response from babies breast tissue. It can be seen in either gender. That would be centrally located & usually soft to hard but non-tender. If it has reddened tissue surrounding it germs may have entered through the nipple ducts to trigger an abscess. A doc could sort this out.