Anyone else get dizzy when they do exercises that involve lifting the head?

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The title pretty much says it all. Ever since I started doing workouts in 2013, I've always been unable to do exercises that involve lifting my head, like toe touch crunches, jacknife crunches, supine push ups, anything like that. My head feels very heavy, and after two or three reps I get dizzy, unless I can put my hands behind my head and forcefully crunch that way.

I've checked my form, made sure my back was flat, made sure I lifted with my abs, made sure I wasn't dehydrated, ate snacks... etc. Would anyone happen to know why this happened? Has this happened to anyone else? If so, how did you stop it from happening?