Hello and thankyou for approaching DoctorSpring with your query.
Please can you answer a few questions so I have a better understanding.
1) When did you have sex and the condom slipped out?
2) When was her last period? how many days did she have her flow for?
3) Usually after how long is her menstrual cycle by which I mean, after how many does she usually get her next period.
4) Was her period unusual after that episode, like dark discharge, severe abdominal cramps, any back pain?

If your girlfriend did have normal periods after that episode, she is definitely not pregnant.. The symptoms you have mentioned are not significant.
Please can you answer my questions as a follow up and I will guide you accordingly.

Thankyou.

Hello and thankyou for the reply.
Your girlfriend is definitely not pregnant as she has had normal periods twice after that episode.
What she is experiencing are normal periods. If one has periods there is no way she can be pregnant the reason being, when a female egg is fertilized by the sperm, it implants on the uterus wall lining. When the egg is not fertilized and not implanted, due to hormonal changes every month the lining of the uterus is shed as periods. When it is implanted the person misses her period.
When a person has her period there is not way she can be pregnant as the uterus lining which is required for the embryo to develop has been shed making it impossible for pregnancy to continue. Even if implantation did occur and she has her period, the embryo is shed along with the uterus wall lining.

The most important sign of pregnancy is absence of monthly periods.
The breast changes are insignificant, and so is the lighter discharge she is having.
You need not worry.

I hope this has helped.
Thankyou
Thankyou