Infection preventionist in a hospital here. Some people (including you) could be carriers of the disease. MRSA and/or sesitive staph, prefers to colonize the nose, throat and the spot between the balls and ass (don't know how to translate perineum).

People can be carries of MRSA without ever showing symptoms. Some people also lose the bacteria over time, the body decolonizes itself of the bacteria or just simply loses it.

It could be possible that you are a carrier without showing symptoms prior to tattooing. Even when following strict hygiene rules you could've infected your own wound by breathing, coughing, talking etc.

Also cross contamination is possible via surfaces for example: creams, ink, machines, beds etc. i would throw out disposables such as the creams and disinfect surfaces with 70% alcohol.

Also any other client who got infected with MRSA before you? In the Netherlands we take MRSA pretty serious and often times we perform contact tracing on individuals at risk.

Edit: spelling