Is there a possibilty of pregnancy

I need more information to be able to answer your question.
How did you both wear "thin pants", but also have penetration?

Were these "thin pants" bathing suits or underwear that were technically being "worn", but were pushed aside? If they were not pushed aside, I am not understanding how penetration could occur.

I will assume what you said about penetration occurring, and assume there was penis-to-vagina penetration without clothing or objects covering either area.

It does not matter how long penetration lasted, if there was ejaculation on/near/inside her vagina during "that process" (I assume this process was penetration?), then it means you had sex.

I have not seen anywhere that you say you and your girlfriend had sex, and it sounds like you are scared and trying not to label it as such.

Long story short: yes, there is a high risk of pregnancy if you have unprotected sex. It does not matter if it lasts for 1 second or 1 minute. If you ejaculate inside (or on/near) your girlfriend's vagina, this is how pregnancy occurs, and there is a high risk.

Did this happen recently? If it has been within 120 hours, your girlfriend can use emergency contraception to reduce her chances of pregnancy.