Your profile says you are only 29 years old. Having cirrhosis of the liver at such a young age is very uncommon as it usually taking decades of injury to the liver to develop stage 4 liver diseases (cirrhosis). Since you are a new mother, perhaps the fluid is a consequence of being pregnant and giving birth? This is a community about cirrhosis and liver disease so I can't speculate about other medical issues.
Note: You would have had many medical tests done during your pregnancy and the birth of your son which would have detected if you had liver disease.
Also please note the “ULQ abdominal pain, back pain, and distension/ bloating” are not typical symptoms of cirrhosis of the liver.
Is it possible this is not truly ascites'?
When we talk about ascites as a complication of advanced cirrhosis, we are most often talking about large amount of fluid buildup in the abdomen. This fluid amounts to liters of fluid and can easily be seen and detected being any doctor with a simple on a physical exam.
When a person with cirrhosis has ascites often there are many liters of fluid build up in their abdomen and the person (man or woman) looks like they are pregnant with an extended belly.
If you had cirrhosis of the liver caused by having some form of liver disease, the MRI would have shown many other abnormalities of your liver and surrounding blood supply that are the results of having cirrhosis of the liver. A person with cirrhosis usually has many symptoms and complications of their liver disease not one. These symptoms include extreme fatigue, low platelet count and other liver related lab abnormal results, varices, hepatic encephalopathy, muscle wasting, jaundice, etc. All of these the gastroenterologist would have looked for during your visit.
They are many other causes of small amounts of fluid around the liver that are not related to liver disease. Only your doctors knowing your medical history and have the results of your tests can make a diagnosis as to what might be the cause of this trace fluid. I recommend talking to your PCP and telling her/him that you are concerned about the results and need an explanation that you can better understand.