What it generally means is that you are not properly grounding and centering before doing magical work. This results in using your personal energy instead of the energy all around you, and that can leave you light-headed, irritable, with a headache, or a variety of other symptoms. If you push enough of your own personal energy out you can even faint.
I would suggest that you get some information on how to ground and center and practice this basic skill until it becomes second nature to you. At that point I think you'll find that you no longer have problems with dizziness when doing spellwork.