Thank you for your question today, I look forward to assisting you. I have nearly 20 years of legal experience in various disciplines, including JAG.
Yes, she is allowed to use the fact that you've bought the item before against it. It's not conclusive evidence, but it is called circumstantial evidence.
If you combine the fact that you've bought the item and the fact that someone is saying that you have huffed the item, that could lead a commander or potentially a jury to believe that you did it.
Now, I won't say that it is strong evidence at all, but it is evidence and can be used against you to try and develop a case for use.
What you should do is stop talking to them. Invoke your Article 31 rights to silence. Many people that only guilty people do that, and that's simply false. When you speak with your command, they are only trying to get information out of you that is helpful to them. If you admit to anything that they can use against you, they will, even if your overall statement is that you didn't use. Their only looking for things that you say that go along with their theory and they'll ignore anything that you say that doesn't.
They have to prove their case, so don't help them do that by talking to them. They always have a way of getting you to admit to things that seem harmless, because they are not admissions of a crime, but in the end it works against you.