If your skin is clean, then there should be no risk of infection from taking bacteria from skin surface into sub cutaneous fat with an injection needle. However, if I've been standing in, or have had my arms in, murky water or a bog, for example (which happens when competing in, or marshalling on, adventure races / mountain marathons / biological fieldwork in South America etc) then I'll avoid injecting into legs or arms, but that still leaves my tummy, which rarely gets so mucky. If there is any mud, then I will use a tiny alcohol wipe to clean skin first. For morning & evening long acting insulin injections, I can sometimes inject after a shower, which ensures clean skin, but that doesn't usually help for mealtime injections.
However, the red bump could be a bruise, rather than an infection? To minimise chances of bruises, best to press on puncture site with a finger, perhaps with a tissue if fingers aren't too clean.
Despite my somewhat muddy lifestyle, and not changing needles every time, I've never had an infection from a injection. I'll be doing the same for this weekend, camping in Peak District, marshalling on a challenge walk on Sunday - I'll do a few of the 31 miles covered by the competitors.
thank you
