My 3 Yr Old Granddaughter Pelvic Appears To Be Swollen

my 3 yr old granddaughter pelvic appears to be swollen on the left side and the area gets harder when she strains. what could be the cause ? she had skull surgery and an implant when she was 4 months due to fluid on her brain and a small tube inserted in the back of her head with tubing draped down her chest into her intestine.