Hi,
I'm 20 years old , single , passed out a large white clot in my first day of period.
I was taking oral contraseptives (yasmin) to prevent my period, could it be the cause?

I don't believe that Yasmin prevents periods but is a contraceptive. Regardless blood clots will be red to brown. White indicates something completely different. This is something that you need to talk to your doctor about. It is weird enough that I haven't heard of it.

Was this mucus like at all, or whitish clot of tissue? Was it with some bleeding or alone?