I had 4+ ketones for weeks in tri 1 due to hyperemesis. I could barely eat and was sick constantly. I had IV fluids every couple of days at home. Some doctors say ketones might be harmful to the baby in super high amounts but that is basically just conjecture, there could be someing in it but there isn't really any particular evidence one way or another. I'm 27 weeks now and our baby seems to be fine anyway.
Low amounts of ketones are not anything to worry about in themselves, they are not thought to be any risk to the baby. If it is something you get regularly though it is important to know why they are there though. Basically ketones are a bi product of the body breaking down fat and protien instead of sugar and carbs. That means a) you are eating a low carb diet, b) you are not eating enough and your body is breaking down muscle and fat, or c) You have diabetes and your body is not able to process blood sugar properly so is resorting to living off protien and fat. If you have not been drinking enough it will concentrate your urine and make it look like you have an even higher quantity of ketones.
If I remember right ketones are produced from what you ate 12-24 hrs before a urine test. I don't think it is possible for you still to have ketones from being sick a few weeks ago. I know mine went back to normal as soon as I could eat normally. Its most likey it has a simple and fairly harmless cause. It is quite possible that you are still eating slightly less than normal though (even without realising ) which is enough to cause some ketones. Or it might be that by coincidence you ate low carbs the day before. For example for some people eating one meal of meat and salad with no rice or potatoes etc would be enough to have some ketones in their urine. If its a low amount and a once off the doctor might not be too bothered. If it is a regular thing or high amount they will probably want to know why.
Hope that helps a bit.