Does allergy get worse when exposed to pollen even if I take antihistamine and feel ok? I mean the underlying longterm immunological cause of allergy

Allergy progression. No one really knows why allergies sometimes get worse with time and often get better over time. Clearly, they can and do change. In order to have an allergy, you first have to have the gene to respond to the allergen. You're born with that. After that, things like bacterial exposures and allergy exposures can cause you to react but there's no way to predict it. Shots can change it downward though.
In some. In some people it does get worse over time, in other it remains stable and eventually it tames down over time in the 5th and 6th decade. Play it by ear.