Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
If your kitten is breathing heavy and low on energy, then this suggests that she is not oxygenating effectively. Animals who cannot get enough oxygen will show elevation in respiratory rate and often will be very lethargic since not moving reduces their body's oxygen demands.
Now there can be a lot of causes for these kinds of signs. Increased respiration and poor oxygenation can be caused by problems associated with the heart, the lungs, and even the blood that carries oxygen to the tissues. So, this does mean a lot of consider for Mia.
Infections in any of these organs could lead to troubles but also even if her gums appear pink, she could be anemic (just not enough to pale the gums yet). This could be due to trauma within the body leading to blood loss or blodd loss due to parasites like fleas (or even blood based ones like hemobartonella). Alternatively, fluid could be accumulating in her lungs (which we'd have expected a positive response to the injection if this was the case). When this happens it is often due to heart based issues. As well, we can see fluid build up in the chest (which prevents the lungs from expanding as they should to take a breath). This can be caused again by heart troubles but also viral infections (ie FIP) and some tumors (and we can see lymphoid tumors in young cats especially if they may be infected with the feline leukemia virus). Less likely here since you've not noted any flu signs, we can see cat flu (including bordetella infections) cause pneumonias for kittens that could manifest this way. And while you haven't noted any coughing or upper airway noise, it is possible to get partial obstructions in their throat if she tried to eat something she should not have (ie piece of toy, bone, etc)
In this situation, since she is very young and kittens cannot sustain elevated respiration for a significant time before becoming exhausted and collapsing (and possibly arresting), we need to be proactive. If she didn't respond to the jabs, then we have to consider that there may be more going on here. Therefore, it would be prudent to consider getting an x-ray +/- checking bloods for Mia. An x-ray will let you see what is going on in her chest. This will tell you if she has fluid or signs of pneumonia in her lungs, if there is fluid in her chest, if there is anything visibly wrong with her heart externally, and whether there could be a mass. It will also tell you if she potentially has something stuck in her airway that could be causing this difficulty.
Otherwise, bloods carry the benefit (especially if the xrays are clear) of allowing you to see if she is anemic and whether blood parasites are present. Depending on your vet's findings, they would be able to target treatment to tackle the cause of her respiratory distress and while doing so keep her breathing more comfortably in an oxygen tent.
Overall, if Mia didn't respond to initial treatments and is breathing heavy, we need to work quickly to identify the causative agent and address it right away to give her the best chance of recovery and survival. So, I would strongly advise revisiting her vet now and if the lungs/heart/chest are suspect getting an x-ray. Otherwise, if anemia is suspected to be more likely (based on what the vet can or cannot hear in the chest), then bloods may be indicated. In either case, we need pinpoint the cause of her breathing difficulties (and get her on some oxygen support in the meantime) to help her overcome this.
I hope this information is helpful.
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All the best,
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