can we cash the checks?

my mother got disability in illinois while she received a bone marrow transplant. she returned to work in march and phoned the ss office in her area to let them know. the checks kept coming. turns out she went back to work too quickly and unfortunately had to quit in order to recover fully. She then contacted her ss office to let them know, and they sent her some kind of form to complete and mail in, which she did. all this time she is receiving a monthly check, which she now desperately needs. can we assume she was never taken off disability, and the checks will continue to come each month? can we cash them without worry we'll have to pay back the money? any input will be appreciated.