When one is manic, the brain doesn't want to stop. Sleep becomes less
and less necessary. Thoughts fly off on tangents endlessly. Ideas can
become grandiose. Often money is copiously spent on unneeded purchases.
In full-blown mania, symptoms of psychosis might start being manifested,
such as delusions and hallucinations.

In my POV, it's natural for you to be wary of feeling well, having been
unwell for so long. You're not used to feeling happy and mentally
healthy, and now you have to come to terms with this unfamiliar
situation.

It's certainly not unheard of for people to be suspicious and
anxious of new-found health after years of chronic suffering. As such,
this might be a very good time to go into short-term therapy, if only to
examine and discover what life has to offer--in particular, to offer
you--without the incessant emotional weight of depression.

As far as the medication you're taking, any psychotropic drug can wane
in efficacy over time. It needn't happen, but it's possible. If such
should occur, your pdoc might choose either to increase the dosage,
improve the effects of the drug by adding another AD to your regimen, or
make a trial of a different AD. But don't worry about it unless you feel
your depression reëmerging.

BTW, it's also not unheard of for an AD to have the effects you're
experiencing on Lexapro. Eleven years ago, after trying sundry other
ADs, my pdoc put me on Nardil. For me, it worked like a miracle drug. My
depression was erased, and astonishingly I've suffered none of the side
effects commonly associated with MAOIs. (I've had spells of depression
since then, for diverse reasons, but in all it remains the only AD that
works for me.) So it most definitely can happen.