Even though your budgie may be active, eating and drinking doesn't mean there isn't a serious problem that needs to be addressed. For the health and well-being of the bird, you really need to have her seen by an Avian Vet as soon as possible. The swollen area may well be a tumor or cyst that isn't causing pain at the moment, but she may also be "masking" any pain she feels as birds do in the wild.
You asked for advice and this is the best advice anyone can give you. When there is a problem or issue of this nature, the best thing you can do for your budgie is have it seen by an Avian Vet for a proper diagnosis and treatment protocol. I hope you will do so right away and let us know what you find out.
Best wishes to your little female.