A decrease in oxygen levels in the blood during exercise is an example of an internal stimulus. It is happening within the organism itself, it will be internal stimulus. But a sudden change of air temperature is not occuring within the body of organism.
It is the change outside so it is external. An artificial light that attracts insects and an increase in the amount of water available to plant. Both are happenings which occur outside the body hence both are external stimulus. Only a decrease in oxygen levels in the blood during exercise happens inside the body so it is only internal stimulus.
