One of a litter of 7 seems to be very jittery/trembling. They are 23 days old. I started introducing bitch replacement milk a couple of times per day yesterday. Only this little girl seems to tremble, i am feeling that the jitters/trembles seem to coincide with the milk replacer but i cant be sure. Afterwards, her nose did feel a little cold, so im wondering if you has just got chilly (although the room is really warm), or whether it is something to do with the feeding? Has anyone experienced this, any advice is very welcome.
She is feeding off mum happily and experimenting with the whelpi milk, and gaining weight.

I don't know what your breed is.  Mine is a small breed (5 to 6 Kg. as an adult).  I have had a few puppies who seemed to tremble as a response to various stimuli, such as first times outdoors, solid food, etc.  I would advise you to check her temperature to be sure it is not abnormally low.  I was worried about the puppies who trembled, but it never amounted to anything at all.  Nevertheless, keep her under close scrutiny.  Are you asking her to nurse from a bottle?  This might really put her off.  Anyhow, why replacement milk in any case?  Doesn't Mom have enough milk?  At over three weeks of age, puppies usually start solid food, not replacement milk.

I DID once have two puppies who, at about the age of yours, apparently had a very adverse reaction to Panacur, stopped nursing, and had such low body temperature that they were really shaking with cold!  I took them to the vet, who was not able to really ascertain much of anything except that they were not limp, apathetic or truly "ill", so I fed them by hand for a few days, really getting the milk into them (with a syringe, not a bottle), and they recovered completely (I never give Panacur to puppies since then, though).

I too am not sure why you are using replacement milk - and if this started after you did with this one, I'd stop.   In any case by the end of week 3 going into week 4, you should begin weaning them onto solids.   We started by using goats milk, warmed and teaching each one to lap by holding them on my knee.  Messy!   Once they could all do this (and we did this four times a day with all their real nourishment still coming from mum).  Once they could all lap, we added a good quality puppy food, mixed into a porridge (either with goats milk, or just warm water) and ditto to make sure each individual puppy could manage it - and that was messy!   Once they could, it was a case of filling our purpose made puppy dishes with the mix, 4 times a day.   Mum was out while the puppies fed, but back in to clean up afterwards and allow 'comfort suckling' - usually standing up and wanting out fast.   Sharp teeth hurt.

ps   The milk should be warmed.

My pups would be getting solids at this age. Is this a large Breed?  A friend had few trembling puppies some time ago and it was a vitamin D (I think... pups had few jabs and no more trembling) but don't quote me on that! I  would take her to the vets if clearly trembling multiple times at that age. Film it when it happens too so you have something to show the vet.