What are the side effects of the medicine you are taking? Research what the side effects are and ask a pharmacist or doctor. Are you underweight? Because sometimes that can cause women to have a light period or missed one. Do go to a gynecologist if you continue having a missed period.
I was on Sertraline for a number of years. It never affected my periods; they came and went in the normal way.
As suggested already, being underweight is one of the most likely reason periods can stop ... when someone gets too little body fat in proportion to her weight, the body thinks it is starving, and stops ovulating and therefore stops having periods. Anxiety can also cause periods to stop. Some young women in college only get their periods during summer break, because they are so stressed at school. The latter seems a little less likely since you are on Sertraline, it seems as though it would relieve anxiety enough that you would keep having your periods.
If you are a virgin, you are not pregnant and don't need to take a pregnancy test. There is no such thing as getting pregnant from a toilet seat, doorknob, towel, bar of soap, or sperm mystically flying through the air. On the other hand, if you are still technically a virgin but have had close contact with a guy's 'guy part,' such as being naked together and embracing closely but not having penetration, you might still have some risk. If you think there was a realistic chance an activity of yours might have made a way for sperm to reach your vaginal secretions, don't hesitate to take a pregnancy test. Your mom's tut-tuts shouldn't keep you from finding out, and the pregnancy tests sold at the Dollar Store are said to be very sensitive. But don't bother to test if the most intimate you have been is innocuous (such as merely kissing, or embracing with your clothing on and pressing together). Those will not get you pregnant, save the trouble of getting a test if that's all you've done.
As suggested above, if your period keeps staying away, see a doctor. (Especially if you are not underweight.) In case you didn't know, you do not have to have your mother in the room when you see the ob/gyn, and the doctor is not allowed to tell her what you have said.
Hope this irons itself out soon!