Opiates effect everyone differently- and the side effects and half lives of the medication can increase interactions/side effects.
That being said - it sounds as if your prescribed dosing is 1-5 mg OxyContin and 1-5 mg methadone tablet TWICE per day- is that correct? Then on top of your prescribed doses- you are self medicating by taking 2 or three times the daily amount in addition- you are TRULY lucky you have not wound up dead or in the hospital from overdosing.
What you are doing is dangerous and very likely to put your life in danger.
These medications are nothing to fool around with , and certainly not to be taken any way but exactly as prescribed.
Methadone has a very long half life meaning precious doses remain higher in your system for 36-72 after taking it.
I am not surprised you are having the reactions and interactions.