Hi,
Thanks for posting in HCM.
I understand your concern.
Based on the description you have given about the act, it is unlikely that your girlfriend could become pregnant.
Although pre-ejaculatory fluid contains sperms, they entering vagina from outside and causing fertilization of egg seems less likely.
If she is on her periods as you have indicated, then it is nothing to worry about as it is not during her fertile period.
Also, if she is taking birth control pills regularly, chances of becoming pregnant is highly unlikely.
However, in case there is a delay in getting periods following this, kindly take a urine
pregnancy test and consult Gynecologist for appropriate management.
Hope the information provided would be helpful.
All the best.