BAD knee pain in both knees...is it arthiritis?

I have been getting bad knee pain in both knees for a few months and it has gotten worse...I have 2 children so I carry them a lot as one is still a baby....I am not overweight at all and am quite fit...I walk on average 2 or 3 miles a day...today my knees are killing me! I have walked about 4 miles pushing a pram...but the pain feels like I have been hit with a mallet...it is there all the time but worsens in waves...so it is bearable for about 30 seconds them it REALLY hurts for 2 or 3 seconds...what is this!!??? I am 36 and female....my sister is 44 and has arthritis but she is overweight and generally unwell with other things,..I did have an eating disorder in my late teens...help? I wll go to the docs this week but wondered if anyone could tell me if it sounds like arthritis?