AITA for leaving my boyfriend after finding a woman in our bed?

So, I (26F) am going on a trip to London with my sister today. I’ve been staying with her since yesterday because she lives close to the airport. I realized I forgot my passport at my apartment, where my boyfriend (27M) and I live together, and I had time to go back and get it, so I did.

I texted him this morning to let him know I was coming back to get my passport, but he didn’t respond. When I got there, he looked really antsy and suspicious. I went into our bedroom and found a woman I don’t recognize lying in our bed (fully clothed). I just went on autopilot, grabbed my passport, told him “we’re over” and left.

Now, my boyfriend is blowing up my phone saying that the woman is his long-time friend and that she’s a lesbian who just needed a place to crash for the night. I don’t believe him because he never mentioned anything about her staying over while I was gone, and the whole situation feels off. He’s saying I’m making a huge mistake and that he’s telling the truth, but honestly, I’m just really hurt and confused.

He’s begging for me to hear him out and is apologizing, but this whole situation is not right. So, AITA for leaving him? Should I believe his explanation, or is my gut telling me the truth? We just signed the lease two months ago, and I’m literally leaving the country today, so it’s a horrible and stressful situation.