I found two possibilities well,it is good to notice changes but if there isn't any swelling, or change of color (blue) and you can feel the pulse-which is located on top of your foot in front of your ankle between the two bones no much to worry.,it cud b due sympathetic-parasympathetic nervous system acting differently on different areas depending upon amount of nerves supplied to dat area.. responsible to blood flow to dat area keeping it warm or cold its a benign condition n rarely get noticed nor require any treatment odr thing which is lil worrisome cud deep vein thrombosis which is a condition whre blood get clots in lower limb veins reasons fr this cud be so many most common being surgery or any sort of trauma.. associated symptoms include pain on stretching,calf muscles pain on lil squeeze.its worrysome since the clot cn break n dislodge in to capillaries of brain or lung vessels blocking it causing acute brain stroke or pulmonary embolism respectively which in turn leads to paralysis of one half of body or immediate threat to life by sudden respiratory failure. dis complications is a medical emergency.u need nt to worry if symptoms r nt dere. to be assured get a colored Doppler of that leg which is a type of ultrasound says abot blood flow in various vessels.