I have a weird indentation about 2 inches long and mildly irritated feeling at the back of my leg, behind my knee. what is this?

Be seen: There are a lot of possibilities. Although it is not terribly likely, a blood clot could present like that and can be life threatening, so I would definitely be seen.

Behind the knee: You stated to have an indentation on back of knee that is mildly irritated. Not sure what this could be based on the information so far. Is it painful, swollen. There is a condition called Bakers Cyst that can present behind the knee.

Can u send pic?: Have you lost weight recently? Normally,dents in the back of your knees is called the popliteal fossae, and it is supposed to be indented as it gives the room for teh major arteries that run up and down the leg, so when you bend , the arteries have space and are not compressed. When carrying more weight the areas around them may seem larger. A pic may be helpful for determining what u may have:)

See doctor: The weird indentation with mild irritation at the back of knee needs to be seen by a doctor to confirm the diagnosis and to be treated