Hi, I'm sorry that you're going through this. These emotions and social situations are tough to deal with and it's great that you're willing to sort out how you feel about this now.
It sounds like you're conflicted about the choice you made. It is okay to feel regret in this way and while it obviously isn't fun, it can be an important and valuable emotion to experience. For instance, it can help you figure out what your future boundaries are, in terms of what you'd like
to do, not do
and what you can change
about the situation in order to feel comfortable
It's okay (and healthy!) to ask for the space that you need from people. Is there anyone at school, like a different friend, who could help be a buffer for you around these people on Monday? Of course, you do not need to tell them the details of why you need support -- just that you're dealing with relationship awkwardness and could use an extra friend right now.
I know I am not the only girl that has done something like this but how is it that I can just be okay with this?
It is okay
to enjoy different types of sexual encounters. The important bits are that you (and everyone involved) are enthusiastically consenting to it and that everyone is being safe (physically, sexually, emotionally, etc). You said you had a fantasy, somewhat acted it out and now you're feeling uncomfortable and regretful. This is not an uncommon or abnormal situation to experience -- you're correct when you say that you are not the only person to do something like this.
Learning about your own boundaries is part of growing up and being a person in this life. You didn't do anything wrong and you can set your future boundaries (and wants!) around this. Does that make sense?
You said that your BF and his friend are likely to ask for a repeat of this encounter. Do you feel comfortable and confident with saying no, if saying no is what you'd like to do?
Some reading for you:
Driver's Ed for the Sexual Superhighway: Navigating Consent
Be Your Own Superhero: Learning How and When to Stand Up for Ourselves
Should I Stay or Should I Go?