I don't get it!!!!

Month #3 TTC #2 & AF decides to show up 5 days early!! Last month I was 3 days late & this month 5 days early WTH! I have a very regular cycle of 28 days, always have. Which is why I don't understand the irregularities in my cycle??! :( I keep hearing how your supposed to be extremely fertile after you conceive. Really? Well that's total BS! I don't get it. What's wrong with me?? Wah.....