Another Ovulation Question?

My cycles are all over the place. In Feb. I know I O'ed around the 14th or 15th because I had blood work done. So to be safe, this month, the hubby and I bd'ed on the 11th, 12th and 14th. None of that time did I get a + on the OPK. I tested for good measure on the 16th and got a + so we took care of business on the morning of the 17th. So my confusion is that then mymonthlycycles said I should expect to O on the 18th. so we bd'ed on 19th and 20th just to be sure. What the hell am I supposed to believe? Do you think I covered my bases?