Hi all need help please new to mirt

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I was on seroxat 40mg for about 3 months had been on 20mg for 3 yrs but I had a very acute episode where I lost it and then the following day I took an overdose which included about 20 seroxat that was 12 days ago since then I have had no seroxat I saw a psychiatrist and he put me on mirtazapine 7 days ago initially I was on 15 mg increased to 30 after 4 days I started my new dose on tues since yesterday i have had extreme waves of dizziness and I feel angry impatient low tearful snappy and have vivid dreams waking every 2hrs covered in sweat can any one help is this withdrawal from seroxat or the increased dose of mirt