Hi there, my name is Sophie and I am a registered veterinarian and I would like to try to help you.
I'm sorry to heat that Alfie has not been well.
From what you describe we can only assume that Alfie has sustained any injury from his jumping up. Injuries do not always present straight away particularly if your dog is excited or there is another adrenaline rush or distraction. Often injurys can appear worse after rest.
The only way to determine what is going on here would be to get a pull physical examination of Alfie with particular attention to him hind limbs and hips. Often no cause is detected and pets recover with a few days of anti-inflammatories and rest. However, there may be a reason your vet can feel like an injury to a ligament or to the patella (kneecap) which is a common cause of lameness in miniature poodles.
Your vet will be able to advise you on the next step depending on the severity of injury they discover (if any) and can range from a few weeks resting to surgery.
I hope this helps. Considering your pet is eating and fine in every other respect you should be ok to make an appointment for Monday.
If you need anything else at all please don't hesitate to reply (it's free to follow up). If I have helped you at all please just rate/accept my answer to let justanswer.com know I have helped you. I hope everything goes well and your dog feels better soon.
All the best, Sophie