What could be the reason of short unbalanced feeling with a feeling of very slight upset on upper stomach ?

By "short": I presume you mean "short-lived" or brief. The autonomic nervous system is wired so that when you feel dizzy or unbalanced a sense of nausea or upset stomach is associated. The imbalance can be due to an inner ear problem, benign positional vertigo, upper respiratory infection, medication side effects, among other more serious causes.If persists >10-14 days, see you doctor for evaluation.Good luck