The keto diet is based around your body being in ketosis. On the keto diet, you seriously limit the amount of carbohydrates (usually net carbs which is total carbohydrates minus any fibre listed on the nutrition label) to around under 20 grams. To ensure you do not starve, it is recommended to increases your protein and fat intake so you stay full and are receiving proper nutrition. Carbs don't just mean the usual pasta, rice, bread, starchy vegetables, sugary sweets and drinks, but also excluded from the keto diet are all fruits, some higher sugar content vegetables, milk, most processed foods, and many many other foods that my 2am brain can't recall. Someone on a keto diet can use test steps at home to test their urines for ketones to confirm their body is in ketosis and is burning the body's fat supply for energy.
Most doctors do not recommend a keto diet during pregnancy and I am not recommending it either. But having ketones in your urine is an indication that your body is using fat stores in your body for energy instead of using carbohydrates from your diet. During pregnancy, if you have not been limiting your carbohydrate intake which does not sound likely given the crisps and banana you said you ate, it could indicate gestational diabetes, poor diet or needing to eat more calories.
While you wait to speak with your medical team, reflect on your diet over the last few days. Have you been eating enough? Have you been getting enough water? Is the food you have been eating of high nutritional value? If you do suspect it is gestational diabetes, ask your medical team what you can do between now and your OGTT to mitigate any risks to the pregnancy.
Hope this information was somewhat helpful.