What Causes Light Bleeding During Menstruation?

I was wearing boxers and cargo shorts. She was wearing khakis and underwear. The top of my body was on top of her and from the stomach down was on the sheets. Shortly after fooling around i came with my lower half stil on sheets. I did not realize my pants were damp so i traveled back up to kiss her and the dampness got on her pants. She felt it and immediately pushed me off and she felt her underwear and on the side of it it was damp, just a little bit on the side. She immediately removed them and wiped herself with toilet paper just in case. We did not have Sex, duing our escapade our clothes stayed on the entire time. She started her period on time (yesterday) and usually she her bleeding is heaviest on the second day. But today is the second day and her period is starting and stopping and light bleeding occurs. Could this be because she was stressing because she thought she was pregnant for a month before she had her period? or is there something else?