Uhh. I accidentally cheated.

Guys I accidentally cheated on the girl I've been dating for 4 months. Here's the deal.

My girl and I made plans for my birthday and when the day came there was no responses to my text messages or calls. We don't follow each other's social media, she has Instagram i have Twitter. There was no contact from her at all for days. Well thinking that she ghosted me I was extremely hurt and basically hit up a woman I had previous relations with in the past. I had sex with her. Well today my gf got back in touch with me and said her phone got water damaged 💔.

What do I do? Do I tell her what I did???