Iv'ing is such a broad topic. Unless you are IV'ing a solution specific to IV, along with using the most sanitary needle / techniques....whatever you are doing can cause literally anything.
If you are IVing street drugs, and methadone, I would not discount it is ripping a hole through your body man. Lungs and heart.
Iits not good to be pumping all these drugs and hitting the gym either. My brother was doing the same shit (minus IV'ing his dose) and broke his back on the squat rack. He is stil paying for it today and after 3 surgeries in, he will most likely need to be on methadone for the rest of his life. You should probably stay away from the weights until you are back in a proper cycle. You don't actually make the best decisions while in this state. It hides pain very well too.
Methadone does something to your lungs as well. Or just your body in general. At least higher doses for sure 1000%. I cycled (or well rode a bike everywhere I went for nearly most of my life) and wasn't until I got onto methadone I noticed my breathing was much different now. I blamed it on the fact I was simply doing more drugs and smoking more cigarettes. But there is certainly a correlation to higher dose methadone and lungs. You simply wind out faster. So it could be muscle related too, and know there are studies with high dose methadone and heart issues. And given your heart is the thing pumping the blood all around, if this isn't working right, everything can have issues because of it.
There are no top athletes on methadone, I am pretty sure

Its a very strong drug and you have to take it easy when on that stuff.
Like I said in all my posts to you before, please stop IV'ing. Go to the doctor and get a full blood work up, get everything checked out. You don't want to hurt yourself and it is only inevitable that you will.
As you get older your heart and lungs become very valuable. You only get 1 heart and 1 set of lungs. Drink your dose like everyone else, and ditch the street dope. Get some good pain pills (eat them) and go herschel walker style (1000 push ups a day).