I am in MMT (methadone maintained treatment) and about 6 months ago I was told that long term methadone use could have an effect on the QT interval on the EKG readings. And ever since then I have been having panic attacks, it seems that after I take my daily dose of methadone I start to get weird feelings,

I. I get a heavy heart beat (like it is my throat)
I have a weak feeling All over
I get kind of light headed, dizzy ,
Then it turns into a full panic attack.
And it seems like I can't catch my breath

I do take welburtion daily so I have stopped taking it to test if it is the combination of these meds may cause these kind of problems, but it did not help.
I do not know how to deal with this, is there any way that maybe i have been on methadone for a long time and my body has become toxic to the medication? Or is this all in my head, I do not know what else I am taking that could cause this kind in of symptoms.

Anyone with some advice or any idea of what I can do about this or how to find out exactly what is causing this.
Thanks for all your help