Here are some common phrases you can use for that:

سرم شلوغ بود

درگیر بودم

کار داشتم
Cheers

درود! :)

That's quite okay.. by the way you could omit the pronoun too, cause we have suffixes at the end of our verbs which reflect the pronoun.. 

مشغول بودم...

Or: سرم شلوغ بود

Good luck! ;)