Er, can you tell us the name of the church? It makes a big difference.
But first, no, you did not commit mortal sin. Even if the matter was grave, grave SIN requires knowledge and willingness as well.
So I’m guessing that the church you went to was Oriental Orthodox, which are called monophysite, which I think is actually not true about them at all. Perhaps, however, they are among the sui iuris churches that are part of the Catholic communion. If the latter, you are free to attend and receive communion there, because they are Catholic and in full communion with the Pope.
But probably they’re not. So you can’t receive. Check and confirm. If they are Oriental Orthodox, I believe their sacraments are valid, just like Assyrian and Eastern Orthodox (not Protestants, though). What that means, if true, is that you can’t receive outside of a serious emergency (you got shot on their porch and needed the Anointing of the Sick or Holy Communion).
Edit: please fact check me to make sure it’s true you can attend any sui iuris Catholic churches just as you would a Latin rite.