There are various resion for fluttering for one side
rib cage. It can be heart issues like murmuring, or
vitamin b deficiency and muscle twitching, it can also be due to abdominal organ issues or after having fall on rib side
Now here you have already done
ultrasound and
colonoscopy and I hope the report is normal so you have ask again here. I suggest to do ecg and tredmile test for cardiac health along with vitamin B complex test to know vitamine B level in body.
As per the report if there is a issue with vitanin B then have it from suppliment range and if there is issue with heart then take treatment as per the need and suggestion by doctor.
Now if all above reports are normal then don't worry I suggest to have good life style and take balance diet rich in calcium and protein. Have proper sleep and do regular exercises.
Here I will also suggest to check with ortho or physio with other possibilities like muscular and
bone pain. And take treatment and exercises suggested by ortho and physio.
I am sure this will be helpful if you follow it properly and do not lift much weight on or with affectes side.
Take care