Of course what you should have done, and don't let on like you didn't know this, no one is going to believe you, is ask the people who administer your SSI about the inheritance. They'd have been glad to tell you.
Why would you be asking the lawyer who handled the estate about reporting it to SSI and why would you ask him and then pretend you did not have to report it to them because of his answer? That's silly. Do you ask your medical doctor how to do your income taxes? Oh well, just don't go into the situation trying to say you didn't know. You did. How many times have you had to sign things saying that you understand you are to report all income to SSI?
What they'd have done, if you had been straight with them from the beginning, is take you off SSI until you had spent all the inheritance on supporting yourself for those months you had it and didn't report it.
Unfortunately, when you are so low income that you are on SSI, you do not get to keep any inheritance or other means of support that you get and save it for your grandchildren, or put it in a mutual fund or something similar. You are supposed to use it to support yourself, so the government does not have to support you. When you run out of assets, then you are able to get back on SSI again.
What has happened, since you did not report the income, is that you are overpaid all the SSI you have received since you got the inheritance. They will have to make a decision about how overpaid you are. They will probably assess penalties and fees for your failure to report this income.
There are thousands and thousands of people who do this very same thing, try to keep getting SSI and not tell them about an inheritance. You have been caught by cross matching, because they watch everyone all the time, and run checks. They expect people to lie and are not overly shocked about it. Yes, you have now gotten a contact, and have been told you may be in serious trouble. But if they prosecuted every single person who did this to the fullest extent of the law, they'd have to build a lot more jails.
Remember, right now, this is NOT the prosecutor in the criminal justice system that you are going to be dealing with. You are dealing with the fraud unit of this agency, and as I said, they see these things all the time.
Since you have not yet spent all the inheritance money, you have ready access to paying it back. That's what they are really going to be concerned with, not punishing you to the max or throwing you in jail and throwing away the key, but getting the money repaid that you received fraudulently. Right now, go on and contact the agency as they have directed.
Be ready and willing to work with them every way you can. Do not waste their time and yours whining about how your daddy was a veteran and wouldn't have wanted you to spend his money this way. They do not care. You lied. End of story. But at this point, you do not need to use your inheritance to lawyer up and try to act like you are being charged with a crime. You aren't, really, not at this point in the situation.
If you ignore them and will not cooperate with their fraud investigation, you are much more likely to be prosecuted for the fraud. If you refuse to talk to them, refuse to admit wrongdoing, as you might do in a criminal prosecution, you'll just slow up and convolute the process.
You did it, this is how much you have been overpaid, they will tell you this, and this is what they are going to do about it. No negotiation, no tap dancing. End of story. But don't pace and scream and whine and carry on. Just work with them and do the best you can to resolve this situation. As I said, lots of people do this, and get caught and have to pay the money back and get sanctioned so they can't be on SSI again for a very long time, and that's what is very likely will happen to you.