Reasons for inverted T waves are numerous and may include ischemia, heart attack, hyperventilation, anxiety, certain medications, infections around the heart, pulmonary embolus (blood clot in the lung), electrolyte disturbance (abnormalities in sodium, potassium levels) etc. Apparently, serious conditions can be ruled out as you would have other signs and symptoms.
The T wave is not isolated but all of the waves are evaluated, not just one in particular. With this in mind and considering that the rest of the waves on the EKG appeared to without any sort of defect, is probably why an abnormal EKG is not considered valid for the past 5 years.