Antibiotics often give this length of remission from Pyometra which is a womb infection. The bad smelling discharge indicates that this may well be what it is and that is is now back. In my experience it always comes back and the only cure is to spay the hamster. Usually when it returns it does not respond as well to the antibiotics. If there is a tumour there is often bright red blood which is on and off. I think you need to see the vet again. There is not always swelling with a pyometra - sometimes just the blood stained discharge and as it progresses, drinking a lot as well as increasing illness with lethargy and lack of appetite.
Some people have had some success with older hamsters and the drug Galastop. You could ask the vet about this maybe. Read Vectis's article and maybe print it out and show to the vet?
Pyometra in Hamsters - Vectis Hamstery