I feel it's important to note that there's also combined type, which combines elements from both hyperactive and inattentive types.
If so, then your inattentiveness could be your mind trying to escape that feeling.
Could it be that you're dreading something that is "implied" by the lecture in a way? Maybe the practise that you feel obliged to do? Maybe the impact the lecture could have on your work or schedule? Maybe just the end of the lecture and back to whatever comes after?