I have a interview under caution!?

ive just recieved a letter saying,following enquiries into the circumstances of my clain to benefit it has become necessary to interview you!,i have dont know why,this is the story...me and my partner split up in july,we lived in rented accomodation,as soon as he left i claimed housing benefit,we got back together in january and he moved back in at the end of march,i informed my benefits straight away,so we now get joint housing benefit,then today i get this letter,but i think i know whats happened,he moved into his mums,who ive just found out gets housing benefit,as shes elderly and lives on her own,now i have nothing to do with my partners bills,and never asked him about bills at his mums,also when i put in the joint claim i had to take his wage slips,which was addressed at his mums!so do you think its him thats being investigated,i did everything by the book when he moved out and i,i changed tax credits also,i cant see what ive done wrong,im sooo scared please help!