Hi there and thanks for your question.
I wish I had your dog in front of me so that I could examine her.
However, what you have described makes me concerned about the possibility of pancreatitis. Dogs with pancreatitis will often do a sort of stretching motion and hold their hind end up in the air in order to try to relieve abdominal pain.
Pancreatitis can be very serious. It is often caused by ingestion of a fatty food, but sometimes we can't find the cause.
Usually dogs with pancreatitis will have a very sore abdomen if you put your hands on either side of her belly just behind her ribs. However, if you are not used to doing this it can be hard to determine.
If you think that she may have pancreatitis it is a good idea to do an emergency visit tonight so that she can be administered some pain relief and started on some IV fluids and IV anti-vomiting medication. Most dogs with pancreatitis are treated successfully but it can be fatal.
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.
There are a number of things that can cause vomiting but they don't usually cause the "butt in the air" stance that you mentioned.
I suppose it is possible that she has a problem with an infection in her anal glands (glands that sit on either side of her anus), but this doesn't cause vomiting usually.
If it is pancreatitis, some dogs can be treated with an injection and then medication to be given orally which is not terribly expensive.
However, most dogs with pancreatitis need to be hospitalized and have bloodwork done and this can be pricey. Depending on how sick she is it could be anywhere from $300 to over $1000.
I hope she does ok!