I Have Been Prescribed Venlafaxine ER, 75 Mg, But Have

I have been prescribed Venlafaxine ER, 75 mg, but have taken clonazepam (.5 mg -- 1/2 tablet) twice a day if needed. My doctor says that it causes dimentia. Of course, I just started my first day ov Venlafzxine ER. I do not feel myself. Is there a reason for this? Am I having withdrawal symptoms from the clonazepam? I originally started taking the clonazepam for panic attacks are my lung collapsed, a month-long hospital stay, and a chest tube for six months.