Hi there - I'm Dr. Sara. I'm sorry that Rudi hasn't been feeling well. I know that it can be scary when they are ill, so I'll do my best to help.
Cats are experts at compensating for their illnesses and hiding their symptoms. One symptom that I never ignore in a cat is any sort of respiratory distress, like abdominal breathing, tachypnea (which means fast breathing, for me anything above 40 breaths in a minute), or open mouth breathing. It's fantastic that you've had her examined and her diagnostics came back normal. It's great when that happens because you know a lot of things that AREN'T going on, but the down side is that you still don't know what IS going on.
When I see kitties with labored breathing and their diagnostics are unremarkable, the disease at the top of my list is feline asthma. Feline asthma is a fairly common cause of respiratory difficulty and it can be life threatening when they have an attack, just like with people. In many cases, the X-rays can look completely normal. The only way to diagnose asthma "for sure" is to obtain an airway sample, which is something usually done by a specialist. However, if I suspect asthma in a cat, often I will treat them with a trail of steroids to see if they respond.
Another possibility for the kitty with respiratory difficulties and normal diagnostics is heart disease. Usually you're going to have abnormalities on a chest X-ray with heart disease, but sometimes they can be subtle. Heart disease can occur in younger kitties, and it is best diagnosed with an ultrasound of the heart, called an echocardiogram.
I wish I could give better news, but I really do see labored breathing in cats as a potential emergency, so I would suggest having her seen as soon as you can, even if it means getting to a veterinary ER.
Please keep in mind that I'm advising you based on your description and my education and experience, which is not a substitute for an in-person exam. Things that are obvious on a physical exam (such as wheezing or crackling heard in the lungs that could indicate asthma or heart disease) cannot be detected over the internet.
I hope that this helps. Please let me know if you have any other specific questions that I can answer :)