My Budgie Seems To Be Feeling Unwell, Help!

[simonep 0]

For almost a week now, my 5 year old budgie is puffing his feathers, he seems sleepier than usual, and his tail is bobbing. He is still eating, drinking, preening, and his droppings seem normal, but his lack of energy is worrying me. I put a heater in his room, gave him some probiotics and vegetables to help him gain strengh, but it doesn’t seem to be doing much. I thought it could be his moutling pass at first, but there aren’t nearly enough feathers in his cage for it to be so. I’m unsure what to do as he still seems alert but still is sleepier and quieter than usual. Should I go to the vet or is this just a phase? (YELLOW BUDGIE IN THE PICTURE)