Like people, when dogs get older, they get skin tags, lumps and bumps that often aren't a problem.
However, a lump that changes should probably be checked out quite soon, just to be safe.
The rule of thumb generally is to keep an eye on the lump and as long as it doesn't change shape or appearance, and is not bothering the dog, it is best left alone.
However, as this lump has changed, she should be checked. The medical examination should consist of a fine needle aspirate which is a procedure in which cells are extracted from the lump to check that there isn't anything bad going on (such as cancerous cells). If all is well, it is safe to leave it alone.
I just want to say that vets are often taught that, if an owner presents with an animal which has lumps and bumps, they should remove them because it's money for the vet and makes the owner happy.
This is not good practice, because a pet should never be under anesthesia for an unnecessary reason. So bear this in mind and if the test comes back negative (which hopefully it will), think twice before you allow the vet to remove it.