Gucci was spayed last oct, she also had a lump. It was not red but it did increase in size. It didn't went away after 3 months, I took her to the vet and it turned out to be a little hernia. She had it repair this feb.
You can check the link I have pics of her lump.
http://www.yorkietalk.com/forums/sho...3&postcount=18
If she is not bleeding or drainage, having fever, red etc you can a while and see if it goes away. Good luck. Please keep us posted.