I have been taking 100mg of methadone while in treatment and I hadn't gone to dose yet, about 30hrs since last dose and I went into full blown panic that sounds similar to your experience. I was pacing, heart racing, horrible anxiety so I got in the car to go to the clinic, literally sobbing the entire way. I almost had to pull over I was panicking too much to drive. But obviously getting there was more important than safety. Anyway as soon as I had the dose I felt calmer but still needed 20min in the car to stop crying and feeling panicked. I mean even just getting to the clinic I felt calmer, it was extremely similar to the feeling of having heroin in your hand.... even though you are still in withdrawal and still need to get the shot in you just having the drugs makes all the symptoms less intense. Obviously that is the mental component, the intense desperation to feel okay and terror of not having it. So I wonder if this is something that happens with methadone specifically, I know that since it has the longer half life the withdrawal begins many hours later than they would with other opiates.... but I didn't have any mild symptoms like yawning and tearing up as a warning just sudden and intense anxiety, emotional, feeling like having a panic attack, but not the other physical symptoms. I have had this happen 2 or 3 more times but never as extreme or happening so suddenly, I felt completely fine one minute and panic attack the next, just that first time.
I am definitely not looking forward to getting off the methadone but I am also incredibly grateful to be off heroin and IV drug use, which wouldn't be possible without it. I'm hoping you didn't have to start at the beginning again, I know how frustrating that feels since I would make it 3/4 through withdrawal just to give in too many times to count. Just throwing away the week of agony by using again. Addiction is crazy like that.