The Herx reaction is your body’s response to riding itself of toxins. This usually causes symptoms such as fatigue, chills, fever, aches & pains. These reactions can last for days or weeks in some cases. This may cause you to initially feel worse when starting a probiotic as they are doing battle clearing out bad bacteria, however eventually these symptoms should subside. If they do not, discontinue the probiotic and try a different brand.
Disclaimer: I am not a medical professional. Using this information is at your own risk.