2 girls 1 horrible situation

I've been dating a girl for about 7 months now we both wanted to take our relationship slowly and aren't official. I'll call her A. Tonight I got a text from someone I haven't seen or talked to in over a year. This girl who I'll B and I had a short lived but intensely passionate romantic entanglement then cut all ties with me. Now she's texted me with a long apology for her behavior and wants to re-kindle the romance. Speaking as a serial monogamist I've never come across this situation before. I know that it technically wouldn't be cheating but it would break A's heart that's something I never want to do. Also I worry that telling her the truth about what's going on would make her think I want to see other women or some other kind of mistrust. I don't know what to tell either of them regardless of what I choose.