I have stage 3 kidney disease and and used a fleet rectal enema for constipation I later found out it may cause kidney failure. Should I be worried?

Yes it can. It contaions monobasic sodium phosphate and dibasic sodium phosphatase heptahydrate which is rarely known to cause kidney failure in certain individuals sometimes, it has been reported but it is very rare, and happens in people with normal kidneys.
See below. What were the ingredients in the fleet's enema? Contact your nephrologist if any of the ingredients are something s/he's told you to avoid. S/he can advise you in this case. In the future, read labels; know what things you should avoid due to your kidney disease. Eat foods with fiber (fruits, veggies) & drink water to prevent constipation.