Severe gastric pain that worsens after taking Calcigen D3 may be due to stomach irritation or acidity caused by the calcium component, especially if taken on an empty stomach. This can lead to pain lasting for hours or even a few days. Try taking it after food and avoid spicy or oily meals. However, since your pain is severe and affecting sleep, it is important to consult a doctor; they may switch you to a different supplement or add a medicine to protect your stomach.