I feel a bit lift headed, is it because I'm not eating enough?

I've been working out more lately and eating less to lose a little weight. I'm 5'8 around 135 lbs and I'm thin but I just want to lose the fat of my stomach so I'll look great in a bikini. When I stand up from sitting down I get light headed and it gets a little fuzzy but then it returns to normal. Today I ran and burned 400 calories and ate 3/4 a cup of oatmeal, a banana, walnuts, 1/2 cup of milk. Then a few hours later I had wheat thins and peanut butter and an apple. For dinner I had grilled chicken and steamed asparagus zucchini and broccoli. Then I slipped and had a brownie with a little ice cream (couldn't resist). Does it sound like I'm eating enough? I don't want to put my health in danger but I want to lose a few pounds!