I realise that this post was written over two years ago, but I have the exact same problem. 2kms into my runs I start to feel it and the pain is disabling, much worse than side cramps and lower down. I also feel like I am going to soil myself but don't, and even when I head home and sit on the toilet nothing comes. Apparently it is due to the oxygen in the blood being taken away from the digestive track to the leg muscles. I don't have low iron and can be fully hydrated and this will still happen. I would breathing related causes. Another thread I read suggested that poor breathing technique and asthma will affect oxygen intake which in turn affects the amount of oxygen available in the blood going to the digestive tract.
Hi there!
Well, without a clinical evaluation it would be difficult to determine the cause of the pain. Possibilities that may need to be considered include lactic acid build up causing cramps, dehydration, hypoglycemia, hernias, metabolic issues, growths/ masses etc. It would be best to have this evaluated by a primary care physician. After a cause is diagnosed, it can be managed accordingly.
Hope this is helpful.
Take care!