need advice

ive been with my fiance for 5 years and im not sure what has happened ! sex life go to the dogs and the passion has turned cold , she has become so secretive her mobile phone goes off at all hours of the night , she has started going out with 'new' friends that i do not know and comes home late always takes and happy i havent seen her naked in 4 months but im sure i spotted a love bite on her neck as she was puting make up on ! i even went through the laundry basket to find her panties there was a white stain on one pair but cant make my mind up on what it is! need help and advice to find out what teh hells going on and if im being cucked if anyone can offer advice then please do

difference between being cucked and cheated on