Tiredness after radiotherapy. I finished in July but I'm still so tired. Some days I can't even shower. It's really getting me down

I had 10 rounds of radiotherapy, chest and brain. I finished on 11th July but I am tired 24/7. My consultant put me on 2mg of Dexamethasone, they worked at 1st but feel as they have stopped. Some days I can't even shower im so tired. Im getting really down about it as im usually a very active person. Can anyone please help. I have stage 3 small cell lung cancer and also had 6 chemotherapy 

Thankyou