Why do i always feel weak and sick when i have a cig?

I have been smoking for 3 years and i have tried to stop but failed. I`m not going to give up and will continue to try and stop smoking till i get their. I used to smoke about 20 cigs a day but now smoke around 5 a day, i slowly cut down over a period of about a year. The problem is when i have a cig a feel really weird, dizzy and tired and it takes me about 40 minutes to recover. This never used to happen and is it normal?