A large soft spot on the back of the right knee ?

Possible cyst/lipoma: You need to see your doctor to make a proper diagnosis. This could be a lipoma or a Baker's cyst. A pulsating mass could be an aneurysm of the vessel.

Baker's cyst?: The back of your knee is called the popliteal fossa. It has nerves and arteries and muscles and joint capsules. You could have a hematoma, a blood clot, a cancer an abscess or fluid collection. If it is not painful then going to the ER tonight is unnecessary. Have your doctor examine it this week and with an x ray or ultrasound you will know for sure what it is. search" Baker;s cyst" on line.

Swelling behind knee: It could be a Baker Cyst or simply a small fat deposit (lipoma). BAKER'S CYST usually are not problem until they rupture. May need you doc to look at it so if there is a rupture in the future he/she may know what to expect. Baker cysts common in patients with arthritis.

Popliteal cyst?: You did not mention how deep the spot you mention is. However, one possibility is a popliteal (or Baker's) cyst. This usually arises from knee problems such as arthritic changes or a meniscal tear. Your doctor should be able to diagnose this using noninvasive methods such as exam or ultrasound.