Achy pain behind my knees, like growing pains, at 20 weeks pregnant?

I'm 20 weeks pregnant and just this last week have had aches on the back side of my legs, a little above my knees. It's dull and feels like "growing pains" I remember from being 12 or 13. My husband and I walk every night and I haven't done anything unusual or strenuous. Is this common? Tylenol relieves the pain but I dont want to take it every day for the rest of my pregnancy. What could this be (causes?), will it last?