Frequent runny tummies, rashes or redness of the skin may indicate contact or food allergies. It can be very tricky to figure out what causes these.
Just because your child ate something or wore something without problems before, you cannot rule it out as the trigger of an allergic reaction. Allergies require "sensitization." They arise only after one or more exposures cause the body to diagnose the allergen as a dangerous interloper and prepare the immune system to fight next time the allergen is encountered.
If you think your child is having an allergic reaction, simplify their diet or reduce the number of things (soaps, fabrics, jewelry, etc.) coming in contact with their skin. Slowly re-introduce different things back into your child's life and watch to see if the reaction recurs. If you are suspicious that a consumer product caused the reaction, you can try to find "secret information" about the product in a safety data sheet online.
For more information, see the links below.
Moving from one city to another can actually trigger seasonal allergies in many people. HowStuffWorks explains the details behind why.