This is exactly what I had and I visited my GP who sent me first for an echo to check for thrombosis which came back negative, and then for a CT scan that diagnosed a 5cm x 8cm lymphocele on the left hand side - there was a small one on the right.
They took me in to hospital and drained the lymphocele inside the CT scanner, taking 60ml and leaving a bag in overnight that drained another 110ml. Released the next day and it has been fine since.
The surgeon said that the righthand side one was pretty typical post RARP and would fix itself.