I would like to know exactly what risks I am taking by having unprotected sex with a girl who regularly uses crystal meth?

It is difficult to answer your question precisely, as the potential risks associated with the use of hard drugs vary. If the person in question does not share the equipment used to consume, the risks are lower than if they use the same equipment as other people. In the latter case, they could be at risk of contracting STIs, such as hepatitis and HIV.