I feel often dizzy.not intensely but a little bit once at a time and that leads to nausea sometimes.what can be the cause of it?

Hard to pin this: down without a good exam. Check BP heart rate and a CBC (blood count) and make sure you're not anemic. That would be the most common cause of dizziness in a 23 year old lady living in a city of 15 million people. If this is not the problem, I suggest you see a neurologist (in the absence of true vertigo and altered hearing the problem may be due to an issue with the nervous system).