gracebunny,
A hemarthrosis (blood in a joint) after a surgery is not due to an infection. It is due to lack of hemostasis, where bleeding continues and it collects within the joint.
If you had developed septic arthritis (Staph is the most common), the drainage from the knee would have been pus (thick, white to yellow, mayonnaise looking material).
Unfortunately, it sounds like during your surgery, the surgeon did not obtain complete hemostasis at the end of the surgery. So, there was most likely a little bleeding vessel somewhere around the joint, which just kept dripping and the blood collected in the joint. Usually, once the knee fills up with the fluid, the pressure will tamponade the vessel and stop the bleeding. Unfortunately, when the knee is drained, it releases the tamponade and the bleeding can start up again.
If the bleeding cannot be stopped by the body tamponading the vessel or from some external compression, sometimes the patient has to be returned to the OR to find the pesky little vessel and tie it off or cauterize it. Luckily, this is usually not necessary.
So, the chances of it happening again are probably pretty low. However, whoever does your next surgery should be made aware of the problem, so that the surgeon can make sure that the wounds are all dry (no bleeding) at the end of the surgery. Orthopedic surgery is usually done under tourniquet control, so that there is very little blood in the surgical field during the surgery. Thus, some surgeons do not release the tourniquet until after the dressing has been applied. So, if they happened to miss a small vessel when obtaining hemostasis (stopping all bleeding), they don't know about it. So, some surgeons will actually let the tourniquet down before closing the incision. That way they can make sure there are no little bleeders that have slipped by. Both of these methods are acceptable and mostly it depends upon the surgeon's training as to how he/she finishes up a surgery.
So, again, the chance of it happening again is probably small. But, it is always possible. Just make the surgeon aware of the fact that you had some problems with post-op bleeding in the past.
Good luck. Hope you get that knee fixed up.