I don't want to scare you or freak you out but to be completely honest here. I would have it checked out asap, urgent care, new doctor whatever you have to do.
My friends 15mo.old had a lump or something similar in her stomach area,along with a the child acting ill, they brought her into the hospital who assumed the parents were abusing the child and they were not and after many tests found out it was a tumor, long story short the baby had cancer, it appeared out of of nowhere very quickly and they just noticed it one day.They said it was just a weird lump on the side of her tummy. The child is now 5 and doing just fine.
When my daughter had a lump on her neck I got in with my daughter's doc. very quickly and my ped. told me she see's this all the time and has no problem checking it out and most of the time it's nothing but she says every once in awhile it is actually something. So she actually thanked me for bringing my daughter in and it was nothing and 3 years later it's still on her neck.