When you were born, you had 1-2 million tiny eggs in your ovaries. Each month, one of those eggs (normally) begins to get bigger and ‘ripen’ a bit like an apple on a branch.
When the egg is big enough, your body produces the LH hormone to help the egg grow. Roughly 24 hrs after this ‘LH surge’ the egg is released from the ovary and starts to travel down the tube to the womb.
If the egg meets sperm on the way, it fertilises and implants into the womb. If it doesn’t, it carries on down and falls out of you, too small to see.
The word ovulation means the moment when the egg is released and starts travelling to the womb. A side effect of ovulation is that your mucus changes from sticky/creamy white, to slippery clear ‘egg white’ for a couple of days. Your temperature also goes up one degree as you ovulate and stays there until this cycle ends.
Meanwhile, your body has been growing thicker womb lining to help feed a baby. If you ovulate, but no egg implants in the womb, the womb sheds the lining and starts growing a new one.
The word period means when the womb lining starts to fall out. This is usually ten days after ovulation. A woman with a 28 day cycle ovulates on day 14; a woman with a 32 day cycle ovulated on day 18.
Ovulation sticks check to see if you had a surge in the LH hormone, when your body told itself “release the egg asap!” But, those sticks can’t tell you if an egg was really released. The only way to know for sure if you ovulated is to either get pregnant, or have internal ultrasound scans to monitor egg growth. In your situation, something is not right and ovulation sticks won’t tell you what, so I wouldn’t bother with them.
So, why are your periods not happening regularly? It means that some part of the process I described above has gone wrong, but I dunno what part. Most likely you aren’t ovulating every month, but it could mean something else. Ask a good gynaecologist.