Sometimes this change will occur when the female is mature and it's mating season.

Other times it may be the result of (as you mentioned) a respiratory infection of some sort.

A few other things to consider:

Parakeets and Budgies have a rather high incidence of malignancies so the very moment an owner notices a lump or bump it's important to have it checked out. Of course this applies to any bird, not just parakeets or budgies.

Even though a tumor may appear on the outside of the body it's quite often because of an inside condition.

Tumors can also be in a male's testes or female's ovaries and there are not always obvious changes until later on when the growth is more dominant inside. The changes you may see could be to the cere or to the legs, even wing area - not necessarily where you'd think they'd be.

Other indications that there may be tumor activity would be a change in cere colo or consistency, weight loss, changes in droppings (often becoming pasty, soiling around the vent) and just subtle, overall changes that owners may sense more than actually see or be able to describe.

By the way, if she's fed a mostly seed diet, this may be contributing to the changes too. A seed only (or predominantly seed) diet contributes to a large number of health problems in budgies/'keets, including fatty liver disease and (of course) tumor activity.

As with all things that might go wrong with our feathered friends, early intervention gives us a better chance at keeping them around a bit longer.