Feeling dizzy after a spell/ritual/magickal practice is most likely due to using your personal energy rather than channelling energy from other sources.
It doesn't necessarily mean the spell has worked, in most cases spells/rituals/magickal practice can take days, weeks, months or longer to take effect, the likelihood of a spell working very much depends on how realistic it is and your own knowledge on the subject.
as Hearth said. Sometimes it could be your mind tricking you or the spell working with you, it really depends on what you were trying to do with the spell, how you did it, and your personal belief. It doesn't necessarily mean it "worked" it just might be a coincidence due to what you were doing after the spell, or just at random.