This may be a gastrointestinal reaction to the calcium you are taking. Calcium citrate is a form of calcium I frequently recommend because it tends to not cause constipation as much as some other forms. But it can cause the opposite problem of loose stools. If you are taking calcium citrate, you may want to try a different form of calcium. The most common form of calcium supplement is calcium carbonate - this is the calcium in Tums and Oscal, among many others. For someone having loose stools, calcium carbonate is a good option. Often, calcium carbonate causes constipation, so watch out for that. Some people do best taking two different forms of calcium - for example, alternating citrate and carbonate. You still need calcium, but may have to try a few different types to see what agrees with you. 

If you are taking magnesium, that can also cause loose stool. You may want to hold off on magnesium until the loose stool resolves, then take a lower dose.