My period... Help please?

I'm 14 and I haven't had my period in almost 6 months , but I get reaaaaaly bad belly cramps , abd I've gained almost 8 pounds? I take baths regularly but my father and step brother use the same bathtub - anything wrong with that?? And as I'm writing this mu stomach and back are cramping,,, I also get light headed , Ive never had , or fooled around , what's wrongg ?