Is it possible to have an undiagnosed dvt behind knee for 8 months with no swelling or reddness? just twitching and dull achy all over leg pain.

Not likely: Well, it's "possible," but isn't likely. A popliteal DVT (vein behind knee) should have swelling & pain below knee at first, less with time. "Twitchy" symptoms really don't go with DVT. There are many causes of leg pain and this probably isn't DVT. 8 month old DVT really not dangerous as clots become stiff like "scar" by that time & won't move. See a Doc to assess possible cause. Hope this helps