Well, I cannot diagnose, by law even in person. Over the internet is harder still expecially without a face-to-face lengthy history & physical.
So... What I am going to suggest is only that a suggestion, if you see in my signature I am not licensed in any area at the moment. So what I am telling you comes only from experience and what schooling I have had so far.
You may have an infection, whether it be a virus or a sexually transmitted one I cannot tell. But from your symptoms I believe it is important you get to a doctor as soon as possible.
I am not sure where you are, but if you are in the states there are doctors that take patients on a sliding scale fee, that means they take only what you can afford.
If you are sexually active it is IMPERATIVE that you have a yearly physical exam to make sure that your female organs stay healthy.
We could be talking about something as mild as simply a missed period (which does happen like this) or something as serious as a ovarian cyst or STD. So, you see why it is important to get this checked out.
You may be able to go to a local clinic or health department, some hospitals that are not privately funded will also treat you. These hospitals depend on private donations rather than sending a bill to someone who cannot pay.