If one undergoes pregnancy, the last trimester creates a hormonal change which loosens the pelvic ligaments. This also affects the ligaments of the foot, at the time when it is possible that the woman is at the greatest weight in life. This obviously compounds the job of the ligaments.
The other factor which creates fatigue is that most of us utilize the calf muscle to propel forward and often use the calf muscle in a lot in aerobic activity. This tightens the calf and creates a greater pull on the heel bone. The effect this has is to pronate or flatten the foot. The small four layers of muscles on the bottom of the foot, and the supportive posterior tibial tendon, must then work harder to maintain the arch. This creates fatigue in the inner ankle, calf and the bottom of the foot.
To combat these forces one must maintain normal weight and continually stretch the calf and hamstring muscles. The next step is to wear more supportive shoes - look for stability labels on athletic shoes. Finally, one might consider over-the-counter or even prescription support devices, prescribed by your podiatrist.