As others have said, it seems like their mom had an uncanny sense of knowing when they were having bowel issues. My mother was like that as well as her two younger sisters. Their mom, our grandmother, was a retired RN and a firm believer in enemas for both kids and adults. And had an uncanny ability recognizing when someone was constipated and was seldom wrong.
When she suspected we were constipated, we got taken to the bathroom, bottoms bared, and placed across her lap. Next, we felt a Vaseline covered finger sliding deep in our rectum. Then her finger moving back and forth twisting as it moved. This continued for a good twenty or thirty seconds. Once she removed her finger, if found not constipated, she offered us a rinse enema that we were free to accept or refuse. If found constipated, she gave us a soapy then a rinse enema, or delegated the job too one of mom's sisters. And like their mom, both girls were good at spotting when someone had issues.
Like us, both girls enjoyed getting enemas. So, as two mischievous teenagers, they used that knowledge to their advantage. Knowing we enjoyed enemas as much as they did, they would tell their mom we’re being fussy or moody, when we were perfectly fine. Because they knew what their mom would tell them, “You know what the problem is. So, take care of it!” Giving them the excuse, to not only give us an enema, but themselves too.
I personally think, girls that are good at spotting things of that nature, simply pay more attention to people than most. I don't know, if they’re picking up on body language, or something else. But my sister was good at spotting my brother and I as teenagers, because she could tell when one of us needed an enema in a heartbeat. And like my grandmother, she was seldom wrong. This led to us swapping enemas with each other whenever she said something about it.