Good morning. I'm so sorry to hear that your girl is not feeling well.
Butter, which is essentially cream, is very high in fat and it's very likely that your girl is experiencing abdominal discomfort right now. Due to the time frame and her desire to go outside, it sounds like she may well be experiencing the beginning symptoms of diarrhea.
If you do present her to the ER this evening, they will likely give you some medication for nausea, intestinal cramping and perhaps some antibiotics to get her GI tract back in order. It's most likely that they will discharge her to you with these meds to use at home, and you'll still have to go through the waiting period where she's going to be uncomfortable for a while. Unfortunately, there's nothing that they can do that will alleviate the symptoms immediately and she's likely going to have to wait out the symptoms until they pass (medication can speed that process some).
Her symptoms are not indicative that she must go immediately to the ER. You can certainly wait until morning to see your regular vet unless you see a progression of her symptoms such as persistent vomiting, diarrhea with blood, belching, distended abdomen, etc.
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