Can adding to much fiber at once cause loose stools, and I've been eating 2 cups of activia yogurt a day plus I have a cold?

Stools. The character and consistency of stools may change with alterations in dietary intake which change the colonic bacterial flora, but as long as this does not change the frequency of stooling, probably will resolve spontaneously if actually due to your changes in diet. On the other hand, the loose stools are more likely related to the virus producing your cold. If it keeps up, see a doctor.