Relationship Between Thermal Resistance and Driving Current

Recently I carried out a experiment where I need to find the thermal resistance of an LED as the driving current increases. As I increase the driving current, the thermal resistance decreases... But when I do the same measurement at constant interval of time (t=1800s), the thermal resistance seems to be increase as driving current increases.. Now I really confuse whether the thermal resistance should increase or decrease with the current..As far as I know, increase in current will also raise the heat power which should increase the thermal resistance. Can anyone advice me on this.. Thanks in advance.