Can diarrhea caused by e coli last for 3+ weeks?

Very rare: That E.coli would cause diarrhea lasting that long. Most E.coli bouts of diarrhea last on the order of five to seven days. The time course you report is way out of the range of normal and I would be suspicious that something else is going on at that point.

YES.: Intestinal infections can temporarily damage the inner lining of the intestinal tract and antibiotics can temporarily disrupt your normal intestinal bacteria, Both processes can prolong G.I. problems, including diarrhea, even after the infection is cleared. Try avoiding fatty, spicy foods & foods that cause gas and try yogurt & other probiotics until your G.I. tract is back to normal.