Heroin Been clean for 4 months now.Former injection site swelling up out of the blue.Advice?

So I use to inject opiates. Mostly fentanyl and sometimes heroin. I've been clean now for 4 almost 5 months. A couple nights back a former injection site I used often, swelled up into a little bubble. I freaked out at first and thought it was an abscess or something but then I calmed down and realized besides from the swelling it didn't have all the other symptoms of an abscess. It didn't hurt it didn't feel hot or warm or anything and it went away after about 15 to 30 minutes. Plus I don't think an abscess takes months to form does it?

Anyway its the first and so far only time it has happened all this time since I've been clean. Does anybody have any experience with this or heard of this happening? I'm thinking its some sort of damage to the vein or something because I had used it so often. I'm hoping to avoid the doctor if possible but if its life threatening or I could lose my arm over it obviously I gotta go see a doctor.

Any advice would be much appreciated.

Thanks.

What colour is it? Does it look bruised, or is it just regular skin but swollen?

Never had this happen. I have been off the needle for around 4 months myself. My tracks are still clear as day but they have never caused any irritation or anything.

And it went away completely after 30 or so minutes?

Just wondering but would you use alcohol to mix/prep the fent with for iv or miss a lot?

You'll be fine.