Hello, Welcome to HCM, Most of these protein powder will have milk or milk byproducts, like whey protein. These will develop an off flavor with time, more so if they are exposed to air. The other ingredients, like flavorings will deteriorate as well. The symptoms may be due to this, because the milk products can upset your gastro intestinal system and may produce symptoms like loose motion. The secondary bacterial infection can leads to constitutional symptoms like fever and headache. So avoid taking this protein powder,take appropriate treatment for the present symptoms. Oral re hydration salt solution Oral NSAIDs Oral Probiotics like lactobacillus. Thank you.