Borewell water colour changes to yellowish black and smells

10 years ago the borewell was dug. So far there is no problem. But a week ago, the borewell water began turning yellow with smell and foaming. After 3 days, it returned to normal but again turned yellowish a few days later. The sewage lines have been checked. There is no sewage line within 20 feet. Could it be an oil flake issue? This is in Chennai urban area with no industries nearby.