What Causes Pain In Back Of The Knees?

i have been working out in the gym for about 4 days a week . half hour on the treadmill and also cross trainer and cycling , a bit of weights etc . about seven days back i felt this pain in my knees and i could not continue running on the tread mill . this pain over the days has become intense . its not bad when i walk on even surface . but using stairs and walking on slight uneven surface is painfull . i feel the pain mainly at the back of my knees and also the inner sides . I am 40 yrs female 5ft 5in , and 68 kgs