En is not a pronoun here, but a preposition. If you leave out the part “surtout que”, it may make more sense:

En ce moment, je réagis très mal aux bruits

En ce moment simply means “at this moment”.

Surtout que is a familiar phrase, used rather in spoken French. It is used to insist on a particular cause or contributing factor. You can translate it as “especially since” or “especially as”.