Painful swelling behind knee. Ultrasound did not detect a bakers cyst or anything else, but its painful and tender, especially in the surrounding area?
Still could be Baker. If it is painful it needs further evaluation. A bakers cyst should easily be seen by ultrasound but sometime the content of the cyst, tissue, synovium, blood, can obscure its deliniation. If x-rays have been ok you may need an MRI but need to consider vascular structures both vein or artery, the posterior knee, cartilage, tumors, etc these are the things to look for.
Hamstring tendinitis. You are probably having some trouble with hamstring tendinitis and that is why the ultrasound did not see it. It is basically inflammation of the hamstring tendons attaching to the outer-back areas of your knee, just as you described. I would re-consult doc for a closer look/exam to confirm. In some case, posterior horn tear (meniscal tear) may cause such pain as well. Consult doc, good luck.