Yes Ashwagandha does not help to increase height and you must consult an endocrinologist. But sorry to say that Ashwagandha does not cause any side effects like diarrhea, vomiting etc. It is a safe drug and has potential benefits. I have published my research paper on this drug after 3 yrs of detail research and before claiming any drug's side effects you need to consult an expert from the particular field.
Ashwagandha will not help you increase your height. An individuals height is completely relative to their body hormones, especially growth hormone. Since your still 18yrs, there's still a chance you will grow a few more centimetres. But if you are very worried about it, then you must consult an endocrinologist, who will completely evaluate your for any hormonal deficiency. And treat for the same if any deficiency is seen. As for side effects of ashwagandha, it can cause abdominal discomfort diarrhoea vomiting. So it better to avoid such remedies