Q. Why are tiny mushrooms growing in the pots of my recently repotted orchids?
This may be caused by too much water or planting material that contained mushroom spores which were activated when you watered. In any case I would start over, clean the pot thoroughly, dampen, not soak, the new planting material before you use it to repot the orchid in. Water and thoroughly drain the new planting and don't water again for at least a week depending on how humid or dry the atmosphere is around the plant.