it doesn't sound like an artery hit to me.
methadone burns like a mofo if you miss it, if you put it in an artery, you'd not have been able to even push the rest of the syringe, it feels like fire under your skin, EVERYWHERE downstream that vein. I can almost say with 100% confidence that you have missed, but not hit an artery.
especially that you said you "felt more pressure" on the shot(s) in question. That just means you were injecting into the interstitial area, not in a vein, not deep in a muscle, just sort of in this area between. I used to be so good at feeling that difference in pressure, I wouldn't even have to pull back on the plunger to see if I was in a vein, I'd know based on the amount of pressure it took to push the plunger down, I would just tap it with my finger, and not even really have to push anything, and if it took more than a certain amount of pressure, I knew I wasn't in and didn't even have to pull back.
All this being said, you should put warm compresses on it, and lightly massage the area, you don't want your body to wall this off and begin attacking it that way, that's how abscesses form. What you want it to absorb it into your body and it won't even have a chance to turn into an infection.
The fact that you're saying it is warm and red tells me the process has begun, so you need to intervene asap. If you can get your body to absorb it, yo'll be fine. If it begins to feel like a little lump, and is tender (painful) to the touch, hot/warm, and begins to constrict muscle movement in that area, you need to hit the warm compresses extra hard, and massage the area, breaking up that lump. You don't, however, want to push that infection deeper into your tissue, so be mindful of that when massaging it.
Draw a ring around the red/swollen area with a marker before you go to bed, when you wake up it should be smaller, if it gets larger thats a problem. If you're going to bed the following night and it's larger than the ring you drew the night before, you're losing the battle.
TL;DR: it's not an arterial hit, it's a miss. warm compresses and massages SHOULD take care of it.
P.S.: I've noticed when in situations like this a few things are critical, eating well so your body has the nutrients to fight the fight, as well as sleeping. Get as much sleep as you can. When you're sleeping your body repairs itself, I've found the best results, ie lumps going down, swelling subsiding, etc, have come from getting a REALLY good nights sleep. Also, DO NOT shoot anything into that area or anywhere near it for a bit, your body doesn't need to fight another fight in that area right now.