What if what you think you are doing is a venial sin but later you find out it was grave matter, does that mean you are suddenly in mortal sin without even meaning to be or would it not be full knowledge if this happened.
No. I believe church teaching is that you have to know it is grave matter in order for it to be a mortal sins. If you don’t know at the time you commit the act, then it isn’t. Not sure if that knocks it down to venial status (I assume it does if that is what you think it is when you commit the act), so maybe someone else can answer that.
The sin is venial because you lacked full knowledge (at the time). But now that you know that sin involves grave matter, if you commit it again now it would be mortal.
Of course, any sin (mortal or venial) can be brought to confession, even though only mortal sins must be confessed. So, if there is even the slightest doubt about whether or not it was mortal, I suggest confessing it anyway.
But you cannot commit a mortal sin if you really don’t know that it’s a mortal sin.