Do you tend to overdo it?

In TCM, it is said that prolonged standing damages the bones (ie. will give you a sore lower back, knees or heels).

Prolonged walking or running damages the tendons (ie. tight hips or general inflexibilty).

Prolonged sitting damages the muscles (ie. makes them atrophy).

Prolonged staring (ie. surfing the net too much) damages the Blood (and hence the eyesight).

Prolonged lying down damages the Qi. 

Do you do one or more of these activities for too long?   Try to change how long you do these activities to prevent further damage or take breaks more frequently.