Hello, I hope I can help you...
I am a bit confused, you say you are a virgin, but are concerned about possible pregnancy? You cannot be pregnant without sexual activity. Were there any sexual relations that you did not mention?
About the bleeding, did you insert anything into your vagina that could have torn the flesh inside? It would not have to be anything necessarily sharp, but the repeated action could be enough to rub it raw. That would be my first guess for the bleeding following masturbation. The only explanation I could think of is irritation or small cut or scratch. Was the blood bright red? That would suggest it is fresh blood, not the brownish blood of a period.
As long as the bleeding stops, you should be OK. I would try another form of masturbation that does not make you bleed. Once you have an area that is irritated or cut, you do not want to introduce anything that could cause you to get and infection, so I would allow yourself to heal for awhile for any thing to clear up.
The change in your periods are likely not connected to the masturbation. In the first years of most women's periods, they are quite inconsistent and change from month to month. For most of us who do not use birth control that regulates our periods, it can be years before your period becomes regular. Some women never have regular periods, but I hope you are not one of them!
Feel free to ask anymore questions, I am happy to help.