The phenomenological theory of dualism argues that the body and the mind are two separate entities, just encompassed into one being. Whether we agree with this concept or not, throughout our lives we will find ourselves subconsciously taking our mind places in instances where our bodies are not fit for the assignment. You could also find yourself bringing your body along simply to satisfy the requirement of your presence, and not for the betterment of your own good. Examples of this may include going to class when you are sick, not canceling on a plan when you are no longer feeling up to it, and my personal favorite, going to work or school when your uterus has decided to rip you apart from the inside out.
As a person with a uterus that experiences menstruation, I have had to deal with uncomfortable cramping and extreme pain since the ripe age of 13 years old, and there is honestly no end in sight. As this is something that is seen as a “woman’s problem” (by the ignorant binary gender system that is forced in American society), it is no surprise that there is little to no leeway when it comes to calling out or not showing up to class because there are squirrels burrowing holes into your inner organs. My cramps were worse when I was in high school, and I can vividly remember moments in my forensics class where I was seconds away from passing out from the pain, but just sitting there as if everything was normal. A few times in the past I have tried to say something about the discomfort, only to be met with comments like, “I get a period too, it’s not that bad,” or “we all have to deal with it.” And yeah, a lot of us do have to deal with it, but why the fuck does that mean I have to sit through a lesson on finger printing when it feels like my body is shutting down? And why have I normalized this exception so much in my life? It isn’t like I am even learning or really paying attention in class, all you can focus on in that moment is the pain. My body is quite simply there just as a placeholder in a chair, not even as a being or a presence.
Unruly Bodies 
It seems like this is a case of ambiguous transcendence. Marx would want you to stay in class because it is an easy display of productivity. In the end, our capitalist world needs people in physical offices and present because it is easier to show people ‘working’. While I can sympathize some with your situation, even I would want a physical body there during the work period. That’s how I get during labs when group members walk out during it–annoyance. While sometimes unfair to either party, it is better to try and get as much done as possible and admit to clocking out to the supervisor. Stand your ground
I agree with @tinaphamm (although I know nothing about theory, I still agree). I think our society places heavy emphasis on seeing the body “work” and being in work mode. I am not sure if you grew up in the DMV, but think of how one of the first questions a college student or adult is asked here “what do you do?” As though standing there, talking to someone, having a good conversation is not valid proof of our sheer existence. I know I derailed a bit, so bringing this back to our cramps, literally, I cannot be in work mode all the time. In some cases, my cramps completely wipe me off my feet and I experience some prettty grueling pain and illness related symptoms. I cannot be productive, but that is okay.
I feel like even though our minds want to see the body working, we also acknowledge subconsciously or consciously that when we do not see the body working, we get it. It is impossible to keep up with the demands! I always felt like this was one of those “Starbucks secret menu” options. It is wrong to punish someone based off of their sex, it is wrong to make somebody work when they are unwell, and it is wrong to push your desires on someone. I feel like at least 2/3 of what I just listed is also illegal and can be reported. But, it is important to stand your ground like @tinaphamm said. Take time to yourself and Care for your body as you need it. If you are not going to be let go from a job, then why not.
I relate to your pain, as my first period was when I was nine years old. The pain ripping through my body would last for days and I would be sobbing, begging someone to help. Not even heavy pain medication could relieve it. It is not okay to be expected to put your body in an uncomfortable situation when you cannot even be mentally present, such as going to school or work when you’re in intense pain. I’m sorry you had to go through that.