I agree, it sounds like it may be a kidney or liver issue. I've had cats with both and they did the same thing. When she vomits is it bright yellow? That is a clear sign it could be the liver or kidneys. If she will let you, check her eyes for yellowing. Also pinch her fur on the back near the neck. If it sticks together and doesn't fall back naturally she is dehydrated.

I understand about the vet bills. I just paid 200 to find out my kitty has a behavior problem but we had to be sure it wasn't anything worse. That paid for the blood work, visit and urine testing. They did a cystoscopy with sedation because he was so upset.