We were at the car dealer and found out my credit score was lower than I thought?

and the salesman asked my boyffriend of 4 yrs out (we live together and all is 50/50) of the blue, if your credit is better will you co sign so she can get a lower interest rate? we were caught off guard and my boyfriend looke flustered and I jumped in, he's very particular of his credit rating and changed the subject. I eventually got the car at a decent rate but now thinking about it, I get upset and wonder if his heart is not into me or if he does not see a future w me. If it were me, i would have said well, lets' talk about it...but of course thats not what he said and not what happened. should I rethink our relationship?