For the past week or so I have had pretty persistent cramping and discomfort behind my knee, on the outer side. I first noticed it around a month ago but then it stopped and didn't come back until now, and now it comes much more frequently, maybe once every hour or so each day. The very first time it came it was a huge cramp I could hardly walk on. After that, it has been a mild cramping and pain behind the knee and into the calf, and a dull ache and feeling of discomfort. It feels warm and puffy at times, and also a little vein back there becomes more noticeable. Due to some other leg pain, I hold that leg very straight and stiff often, so that is possibly related. One doctor said a possible baker's cyst, but I do not have any visible lump behind the knee ever. What could it be?