This is the 13604th most frequent Russian word.
недомогание
A feeling of slight illness or malaise.
Here, 'недомогание' refers to a mild state of not feeling well, physically unwell.
Чувствую лёгкое недомогание, похоже, простыл.
I feel slight indisposition, it seems I caught a cold.
In this case, 'недомогание' signifies a vague sense of discomfort possibly caused by fatigue.
После долгого дня у меня началось какое-то недомогание.
After a long day, I started to feel some indisposition.
Here, 'недомогание' is used as the object of the noun, meaning a specific condition of feeling unwell or unfit which is being investigated.