What is the temperature of the water you are using? It should be no hotter than 103 degrees F (39.5 C) at preparation time, and no cooler than body temperature.
Also, don't hang the bag more than about 2 feet above your hips or it may cause uncomfortable cramping. Allow a few ounces in at a time while breathing slowly and deeply, and if you start to feel too full or if you start to cramp, pant like a dog using short shallow breaths and clamp off the flow. After a minute or so when the cramping subsides, resume the flow.