i have herd that it is bad for rats i don't know how she got it, is it contagious can it be treated??

Put it down.

Only feed her the regular food she gets for today, don't give her any snacks. It could just be something she ate and should go away in a day or two. If it doesn't go away call a vet, they can probably give her some medicine for it.

If she is seriously a baby (like still nursing), then I would just leave her alone and see if it stops. I don't think it is contagious unless she is sick with something more serious and the diarrhea is just a symptom.

Rats cannot vomit. It is possible that she ate something that did not agree with her and she is simply passing it. If she is weaned, try feeding her some oats or a little oatmeal. Maybe a little rice. Make sure she is drinking water, and eating. If she has diarrhea for more than 24 hours, call a vet. pets question and answers,www.5d2d.com
Also, check the food you are using. If it is a seed mix or a homemade mix, there may be too much fat in it, or she has been picking out her favorite bits and leaving the rest, resulting in a nutritional deficiency. Rat blocks, also known as lab blocks or rat pellets, are the best, most complete food. It contains everything they need. If she was very recently weaned and it is still difficult for her to gnaw the blocks, you can try soaking a block in a very little water and making a soft food out of it. This is also a good way to sneak some fluids into her if she is eating but not drinking.
Hope she gets better soon!

Rat owner
Veterinary student