How come? Help!

Im 24 years old. I've been trying to get pregnant for 4 months. I eat very healthy, eat lots of veggies and fruits, I don't smoke nor drink, taking prenatal vitamins the last month, have sex on ovulation day, don't drink caffeine, what am I doing wrong? Im young and healthy.. Why is it so hard to conceive? Oh yeah and I got my period today :(