Many things. We all have good and bad bacteria in our system and we all have yeast in our system. For whatever reason something can trigger either one of these things. I would say that since you are becoming sexually active this maybe a trigger. By doing so you have entered some forgien substances into your body and your body maybe reacting to it. Also if you are taking antibiotics that can trigger yeast. Somethings that you can do to keep this from happening is make sure you stay clean and dry in your vaginal area, eat yougert because it has the good bacteria your body needs, wear cotton croutch panties because they do not trap moisture, and use protection. While neither one of these things will last a lifetime, they can reoccur from time to time and be very uncomfortable to deal with. So see your doctor and stop suffering.
What causes yeast / bacterial infections?
Sounds like you have a yeast or bacterial infection. This is not uncommon when begin sexual activity, nor is it a hard problem to solve. You need to either go to your doctor for a test, you can by a test kit that will tell you if it is bacterial or yeast. Generally you can use an over the counter cream such as monostat and it will take care of it. If it is bacterial, you will need medication perscribed by your doctor.