Thanks for reaching out to Ask Lafeber.
A swollen lower belly in a female parakeet can be serious, so it’s good you noticed it early. One common cause is egg development, especially if she is mature—she may be carrying an egg, which can cause a round, firm swelling low on the belly along with behaviors like spending more time at the bottom of the cage or trying to nest. However, if she is unable to lay the egg, this could become egg binding, which is a life-threatening emergency marked by straining, lethargy, puffed feathers, tail bobbing, or not eating. Other possible causes include infections or inflammation in the reproductive system, tumors or cysts (especially in older birds), or fluid buildup, which may make the belly feel soft or squishy. You should monitor her closely by checking if she is active, eating, and breathing normally, and provide supportive care by keeping her warm, minimizing stress, and ensuring easy access to food and water. If you notice signs like weakness, labored breathing, sitting at the bottom of the cage, straining, or a lack of appetite, you should take her to an avian vet immediately, as birds often hide illness and this condition should be treated as urgent until proven otherwise.