I am able to do dumb bell curl with 25 pound but if I don't do exercise for 3 weeks, i am still able to curl 25 pound but now with pain afterwards, i am assuming hypertrophy, This repeat hypertrophy y loss of muscle and gain of muscle ,Will it help muscle easy hypertrophy in future?
Understand the first month neural changes not hypertrophy are responsible for your strength gains.
Your nervous system is rearranging as it is acquiring new muscle activation and recruitment patterns. You're initially forming the "skill" (Notice systemic changes are typically seen first -- slightly increased vascularity, improved coordination, i.e. the lifts don't feel nearly as awkward as you get used to them).
The weight you're lifting and the extent to which it's challenging for you is relative, looking at it from an absolute stanpoint 25lbs will not be inducing much if any hypertrophy.
Age, genetics, diet, recovery etc, etc.... all play a huge role in how much muscle you're actually putting on.
Unlike cardiovascular exercise which leads to a fairly fast and significant level of detraining (again this level is relative and depends on your baseline level of fitness).
Hypertrophic gains require much less subsequent exercise to maintain (some studies show as little as once bi-weekly to maintain gains) muscle mass also takes much longer (~3x's as long) to lose once you stop completely.
Having pain (or namely DOMS) after a workout doesn't mean that your muscles are going under hypertrophy, it's just DOMs, it won't cause any extra hypertrophy and it is actually detrimental that you stop in between and then do it after 3 weeks.
It is a frequently asked question by new people and I would refer you to this article by Lyal McDonal who shows that DOMs is not an indicator of muscular growth, in fact, it is a factor of detrimental growth.