1 tsp in a glass of water should not have caused you any problems. When people do a Liver Flush they take considerably more than that. Epsom Salts is just magnesium sulfate...however, it is important that you make sure you use the kind that is safe for internal ingestion. Generally ES will not cause cramping or burning. If you are terribly constipated it may have caused some cramping as it draws water into the bowel to assist in elimination.
Since you had an adverse reaction, I would suggest that you perform an enema...PURIFIED water with a bit of Sea Salt added...to clear out any waste.