I have been accused of food stamp (SNAP) fraud!! someone turned me in saying my boyfriend was living with me!?

I am so stressed about this! I have never been in trouble for anything and im pretty sure its my boyfriends ex that turned it in cuz she has been trying to get money from him and realized she could not. I received foodstamps for 7 months but it has been 8 months ago, I didn't want them anymore so I stopped them but still received a medical card for my son. Me and my sons dad was split up from before I even found out I was preg until just 2 months ago. He would come visit his son aat my house but he lived with his mom and a few friends for the past 2 years although he has been using my address cuz he did not trust using his moms. When it was time to recertifiy my sons medical card which was approx 2 months ago I filled it out cuz we were still split up but the next week we started talking n working things out well I received another letter for more info for medical card, I did not fill this out cuz I figured id just let it go since me and my bf were gonna try and let him move back in. Well last week I received a letter that someone reported me for him living with me but I stopped the medical card and everything. I called and they said they will be investigating but how in the world can I prove he was not there when it all started over a year ago!! Gosh im so stressed someone please help! Thanks!