Your thoughts may be right.
My suggestion for you would be:
1. Please take her back to the surgeon who operated her or to his assistant.
2. Get an ultrasound
of the abdomen and X-ray of the KUB region done.
3. Anything suspectable can be seen on these 2 investigations.
4. We need to also check if the postoperative wound is healing well or not.
5. Re-check on the antibiotics that she is on. Is it really helping her?
Hope this helps you a bit.
I wish your sister a speedy recovery.