"From a clear skin to acne after vitamin B12 pills, could there be a link?"
Possibly but this is has only been found for vitamin B12 injections only, not oral supplementation.
There have been a number of studies looking into this and it was first reported in 1967 as a single case report! This is only reported generally in a case report type way so we don’t have any real numbers of it. In one study of 10 subjects who took vitamin B12 supplements (all via intramuscular injection, not orally) one person developed acne within one week of starting.
So if you think your oral vitamin B12 is causing you acne, if you don’t need to take it for health reasons, then perhaps consider stopping it.
However, there is no evidence pointing towards the oral supplementation of B12 causing acne, just the injected ones.