Aspertame is scary... I just finished a book by a Candace Pert PH.D. Who is a research scientist that did her own study on aspertame. It's made from two amino acids, phentlalanine and asparic acid. Her finding were that attached two those two acids were mesthyl-ester which becomes methanol in the body. When methanol is broken down in the liver it turns into formaldehyde. According to the Merck Manual, formaldehyde had been listed as a carcinogen by the EPA.
It could be the cause of swelling on your legs but you'll know for sure when you stop drinking the stuff. I have drank diet sodas for years and have since stopped since reading that book. It's only been two weeks but I firgured I drank it so often that I was numb to some of the side effect. Who really knows but that information scared the piss out me. It was good information for me and I hope it help you.