Not much forethought involved, but I knew what i was saying I guess as I said it.
Sounds like a knee jerk reaction which would not have the three conditions for Mortal Sin unless you are in a cloistered monastery and have taken a vow of silence. Maybe not even then, I would talk to your abbot in that case.
I don’t know about the degree of guilt, but according to one of the ten commandments we are not allowed to take the Name of God in vain.
Are you going to take our word for it if we say it is ?
I would ask a priest.