Vegetarian/Vegan

So couple months ago, i decided to stop eating meat and animal products. I changed my diet to all veggies and fruits/nuts. I felt great for a couple of weeks and then started noticing i felt a little tired during the day. I have been to the doctors and they said everything looks normal. So my question is where do you vegans and plant based diet doers get your extra nutrients from? Also should I buy vitamins, should I get iron pills and what kind?