Daisy - Ruddy heck! Belly button piercing in Bangkok! Not a good idea, unless you saw the needle package being unwrapped in front of your eyes, and even then, more likely to get a skin infection in a hot humid country than in UK. I'm not trying to scare you, but if the needle wasn't sterile, there is a chance you could have been exposed to a blood-borne infection eg Hepatitis B (for which an immunisation is available), Hepatitis C or HIV (no immunisation available).
Diabetics' wounds don't necessarily take longer to heal, as long as they keep their blood sugar levels under control, but infection can make levels go a bit haywire.