This is a great question and one that can have many answers!
There are many reasons why symptoms may worsen or lessen from one cycle to the other:
Stress: Stress can worsen symptoms. If you are having a stressful month, you might notice that your premenstrual symptoms are heightened.
Changes in lifestyle/habits: Shifts in your schedule, sleep, or habits such as exercise can make you more vulnerable to stress and may worsen PMDD symptoms in any given month.
Diet: If you have been eating lots of high-sugar or salty foods - or drinking more caffeine, this could potentially affect symptoms.
Substance use: Using alcohol or drugs can make symptoms worse.
Medications: If you have skipped doses of a prescribed medication that you take for PMDD, this could make symptoms worse.
No ovulation: If you have had ‘one good month and one bad month’, This may be due to having an anovulatory cycle. This is when a menstrual cycle does not result in ovulation. That is to say; your ovaries do not release an egg.
You can read more here: What is an anovulatory cycle?
Tracking your ovulation and any stress/lifestyle changes can be useful for figuring out why your symptoms may be improving/worsening so that you can plan and make decisions to ensure you are not suffering more than you need to.