Very confused

hi im 27 years old woman .im married for 8 years .i got an awesome husband..i added one guy in facebook after my marriage,he s my friends ex boyfriend .very flirty guy he s ,used to chat with all married women ..he said he likes me .so we chatted for some time in whasapp ...after that he got married ,nd i too avoided him ..bcoz i dont like his character .after some days he messaged me nd i too replied nd our chatting again went for some days ... and here started the confusion ,whenever he comes online in whatsapp my neighbour also used to come online same time ,she is married nd have 4 kids ..i dont know how they got contact ,,i asked that guy abt this ,,he said no ..he dont know her , but one day me nd my neighbour went out ,nd i gave my mobile to her .nd went to rest room for sometime ..may be that time she checked my whatsapp messages ,nd took that contact ..bcoz after this incident only they r coming online same time ..but whenever he message me ,,she also comes at the same time ..nd i put status .that u stole my contact nd its really shame on u ..after that they came online once r twice like that ..den she dint open her whatsapp for 10 days ..nd now she opened ,,but they r not chatting in this number ..may b in another number ..i dunno that ..plzz help me ..am i confusing myself ..r it may be true ,,pllz pllz help me