Gf told me she cheated on me

My gf suddenly, out of nowhere, texted me that she cheated on me with a man whom she met for just 2-3 months. We’re together for around 3 years now. What should I do? She’s busy working and her day ends in 12am so can’t talk right now. She was sorry and was in guilt and just wanted to confess as the burden would be less. Is she joking around? What should I ask and do? Your opinion matters!!!!!!