I always feel so wordy when I reply to these things but...
You could try checking your intake using a calorie counter. I think Fitday and Prevention's Healthtracker (what I use) are the same. You plunk in everything you eat and it will give you the breakdown of vitamins in your food. You can see where you are lacking. I surprisingly get way more B vitamins and iron than I realized and other vitamins are more lacking. Because I eat so little dairy, even with the multivitamin, I am still below 100% for calcium and osteoporosis runs in my family.
Speaking of multivitamins, about a month ago I started taking them everyday, which is amazing, I am not a creature of habit. Around that time, I had a pregnancy scare. I was experiencing morning nausea and dizziness. Man that was scary. I was a week late. When TOM came and I was still having morning sickness I seriously investigated and found out that my daily multivitamin was giving me morning sickness!!!
So right now I don't take it. I don't know what to do actually. I've experimented a little and taken it and been fine one day and then the next day I practically have to run out of my classroom and barf out the window. It's too bad, I have faith in vitamins!