You mentioned you had become somewhat inactive after achieving your goal. To me that was not right from you. You should have continued with the exercise along with the normal diet.
It varies according to the individual I guess. If you have more fat, you will need to exercise more for obvious reason and vice-verse. And once you achieved the goal you need to exercise still in order to maintain your body.
If you're not having this issue while running outside, its not a big one. And it is mostly because of the treadmill. There is nothing to worry. Continue with your normal running schedule.