With periods this irregular it is likely that you don’t ovulate, at least not every month. 21 days between first days of periods doesn’t give enough time for ovulation to take place. For cycles of more than 28 days, you really cannot tell whether you ovulate without a blood test taken seven days before the bleed.
This is, of course, very difficult to organise as you don’t know when to take the blood sample until it is too late. Sometimes a sample is taken every week or every few days. In any case, it only tells you whether you have ovulated or not in that particular month. It says nothing about the next month. In order to avoid all this, most fertility specialists would probably prescribe a fertility drug such as clomiphene. This should give you a regular period and make sure that you ovulate each month.