My boxer has a pea sized red lump under her chin

Hi Lindsay, Boxers are famous for lumps and bumps as you probably know.  If you think it is growing larger, than I would have it checked by the vet.  There are so many different possibilities like a sebaceous cyst, a wart, a benign tumor or a cancerous tumor.  The vet may be able to tell just by feeling it or they may want to take a fine needle aspirate which is just done by inserting a needle into the lump,  removing some cells and placing them on a slide to examine under the microscope.  I always feel it is better to know and take out the guess work.  Good luck!