I have had a credit card of my moms for years (I moved away to go to college) I am not an authorized user but have made several purchases and paid them off such as tires and a washer and dryer and several other items. I have always paid it off. Its had a zero balance until recently. My mom passed away and I accidentally used it a couple of times out of habit not thinking about it being in my moms name at the time. I pay way above the minimum monthly amount and I do plan to pay it off. How much trouble am I in with the credit card company."

Stop using the card right now, the credit card company can have you arrested for fraud and/or identity theft even though it was your mom's card. Did your mom have a will and was there probate?

Yes, cut up that card now. Make sure you pay it off ASAP.
What you did ( or are doing) is against the law.