Going to the doctor, not sick..?

Okay so I'm trying to get out of work tomorrow to go to a concert. This is a once in a lifetime thing, the concerts free, my two favorite bands & theyre doing a meet and greet, I have to go. But it was just announced yesterday & I have to work. My job has a policy about sick days, when you're sick you're suppose to bring a doctors note or you're fired. I don't know what to do, I was planning on blaming my period & then going to the appointment to discuss birth control options, I was also going to go with stomach cramps & diarrhea. I'm not sure though, at this point in thinking period is the best way to go. Also, when they test my urine, can they tell that I smoke weed? I'm a pretty regular smoker, but I'm not my parents insurance plan so my mom will come with me to this appointment & I can't have her knowing about it.