If she was living with you she should not have had a housing benefit claim (or any other means tested benefit) as a single person, so your name should have been on all such claims. I assume she has moved out with the children & that you are on your own so not entitled to benefits. As the last post said, contact the authorities & tell them of the change of circumstances - otherwise you could be held liable for the overpayments as well as her.
If she was claiming benefits as a single parent (i.e. without naming you or anyone else as her partner) when she was living with you that is likely to be seen as fraud, quite apart from what she is doing now.