I observe very strong sensations even post meditation. They are strong at some time and some time subtle. I am alternating between apanana and vipasana. I would like to understand why they are so strong at times.
The experiences are based on the sum total of your past fabrications and what has surfaced in a given time.
In the different times the fabrications that has surfaced are different hence the diversity of the experience.
Dwelling on matters like this will inevitably make you crave or be averse to some types of sensation hence regress in your pratice. You should just be aware regardless of if it is strong or not.
Having said this some times the strong sensations can be due to rapture.
Vipassana meditation, if practiced correctly and consistently, will reveal the true nature of reality for the meditator.
What you are describing and have seen through your practice, is the impermanence of conditioned phenomena. The characteristic of impermanence is one of the Three Marks of Existence.
Bhikkhu Bodhi gives the example of a waterfall that from a distance looks solid but when taking a closer look one can see the flow constantly changing.
Vipassana meditation is when one is taking a closer look at reality, which reveals e.g. that sensations do not have a fixed intensity, but instead they change constantly - sometimes they are strong, sometimes subtle.
That is the nature of condition phenomena. They are not constant. They are subject to alteration, change and decay. They are dynamic processes and not stable entities.