Help, Ex trying to sue for 7500 dollars

Please help, I live in the state of colorado. A ex boyfriend is trying to sue me for 7500 dollars from 3 years ago in september. He had given me the money when he moved out of state and I had paid off a joint credit card of 2800 dollars with it and used most of the rest of the money towards another joint credit card.
He had given me the money in the form of a check and no kind of written agreement, or verbal agreement was said regarding a "loan". We broke up a year later and until today he had never even mentioned a word about the money. I was served papers today saying he was suing me for 7500 dollars. I have been paying on a credit card in his name "I am the secondary cardholder", for about 2 years sending out a total of about 2000 dollars. Mysteriously I rarely receive statements and the latest statement i received said the card was still closed to maxed out.
How is it possible he can sue me for this money when it was never a loan to begin with?? Any answers appreciated, thanks.