Hola Angie,
Ok, I'd say "Es lunes, estoy de nuevo EN EL trabajo"
or
"Es lunes, de nuevo EN EL trabajo"
Except for the preposicion "a" everything's correct.
And as you can see, you can ommit "estoy".
Regarding "de nuevo", this is a "locuci�n adverbial" that's an advervial idiom, or group of words working as an adverb. So, "de nuevo" is the equivalent to the adverb "nuevamente" or "again".
Note that in this case, you could also translate "back to.." as "de vuelta": 'De vuelta al trabajo', but the translation you gave is correct.
Hope this helps!