My girlfriend is currently about a week late on her period, and I appreciate there are multiple things that can affect a period, basically anything that disrupts your hormones, but just wanted some advice/opinions on how likely this could cause it.

So the only time we had sex between her last period and now, I came and kept going for a short while (30secs - minute) whilst still hard until she orgasmed too. As I pulled out I felt the condom start to come off me. The condom didn't fully come off my penis until I had fully pulled out. About half the condom was left in her whilst the open end was dangling out of her.

How likely in this scenario is it that cum could have gotten out from within the condom and into her?