Was My First Love Cheating?

We are dating almost for six months. She was my first love, and I loved her deeply. We used to talk on the phone a lot. One day, I called her, but she didn't pick up. Since she always keeps her phone with her, I worried and kept dialing. After calling five times, she finally answered. I asked her what was wrong, but she couldn't talk. I heard heavy breathing on the other end. She claimed she was at the gym, jogging, and that's why she was breathing heavily. Then, she abruptly ended the call, and right after, I heard a man's laugh. I tried calling her again, but she didn't answer for thirty minutes. When she finally picked up, she apologized, saying there was no signal inside the gym. I'm incredibly suspicious. The heavy breathing and the laughter in the background make me think something else was going on. There's a clear difference between breathing heavily from exercise and breathing heavily from pleasure. I'm confused and don't know what to do. She was my first love, and I can't seem to move on. I'm feeling destroyed by this situation. I desperately need some advice.