Kudos for the guts in admitting being such a sheep, but there is no such thing as 'water poisoning' and which would cause a side ache for months. Too much water can cause hyponatraemia, which is real, and would cause symptoms such as headaches etc, and is often fatal- in races. Your side stitch was probably caused by something else, excacerbated by the full stomach
FYI, read up about it, more runners in ultras (study at Comrades) have died from hyponatraemia (slow pace, drink often, too much water) than dehydration which is self limiting before very serious consequences (i.e. you cannot run any further, then drink water and generally ok)