Now i walked for 30 mins at speed between 4 to 6 on a treadmill. but when i finished i was dizzy with a slight nausea what could be the reason thnx?

Two thoughts.: If you've just started exercising, you could just need to be better conditioned by continued exercise on a near daily basis. If you've been exercising, this sound like you may just need to drink more fluids with your exercise, before and after, as exercise could dehydrate you if you don't drink enough fluids. If if continues, see a doctor for some blood testing.

Several things: If you were a 40 year old male with a 20 year smoking history, I'd say you need to see a cardiologist and treadmill test 2 rule out cardiac ischemia. However, you are a young female, so you are very low risk for having a "heart attack" This could be dehydration, postural hypotension, or even symptoms from an oncoming viral illness. Start with your primary care doc for assessment! Hope this helps!