Is Following Vegan Diet Along With Vitamin Supplements Safe?

HI, I went vegan for a few months. I thought I was getting enough of all the required vitamins and minerals. I took a multivitamin for vit D and b12, iron and folic acid, for those were what i feared on being deffecient. also i had protein sources like nuts, beans, tofu, seitan, and vegan protein powder. After a while my tongue was hurting and my mouth was swollen. started to eat fish and took more b12. it got better. also I would work out and I was very weak, and recovery was slow and painful. so i ate eggs again and feel better. now I want to retry veganism. what did I do wrong. what was happening to me?