Do you think your grades can handle your absence? If it can, then, by all means, stay. If it can't, then you've got to force yourself up and walk. What's got your stomach in pain though? I hope that it gets better for you.
My first class was a lab (my prof. doesn't take attendance on those days). My next class allows a total of 3 unexcused absences before it starts affecting my grade, and this is my 3rd (and final) one, so it's good. For the last class, this would be my first time missing it, so no harm done there.
As for my stomach pain, eh, it's good 'ol menstrual cramps trying to creep up on me. They're not as bad this time, though. It might be from the fluoxetine I'm on, or perhaps keeping in shape for the past few weeks has something to do with it.