There are a lot of possibilities. She could be pregnant, mass in the abdomen, ascitis(fluid in the peritoneal cavity).. etc.

Take her to a surgeon.

Hello
Thanks for your query.

I will try my best to provide you with best possible professional recommendations to your questions.

There are many condition which can present with lower abdomen masses which includes Appendicitis or appendicular mass, Colonic Polyps, Colon Carcinoma, Cecitis - usually secondary to Crohn's Disease, or it might be coming from the ovary, may be cyst or tumour.
you have get her scan done to know the cause of mass.
I advice you to consult OBG specialist or surgeon.

Yet again, I duly appreciate your query to me, I do hope that you have found something helpful and I will be glad to answer any further queries.

Regards