intermittent Light-Headedness

I'm a twenty-year old female with very mild hypothyroidism (although I haven't been on medication for a few months).
I'm not on any other medication or oral contraceptives. Recently I have been taking a mild antihistamine, however.

I've been having episodes of light-headedness and dizziness that happen a few times per day (up to seven episodes) and last a couple seconds.They haven't been severe enough for me to completely pass out or K.O. I do however stumble around and the best way to describe it is to say that a blackout for a split-second. Its kind of like I'm EXTREMELY drunk for a this time.

I am not malnourished. I eat quite healthy, although I do over-eat. Also, I haven't been taking my thyroid meds for over eight months- these episodes have only been happening for about three weeks.