A lot of good advice has been given here. I would suggest following the advice from Taker Park.
As a US citizen, I've had my share of hassles with secondary US Customs, primarily at DTW.
I've been asked questions demeaning questions like that, but in reference to drugs. I would always ask, "What do you mean?" or respond with a question when a blanket, open ended question like that is asked of me.
To this day I don't know why I was always flagged for secondary on every entry over a one year period of time or so, but I have a strong suspicion. Around the same time I had all the entry problems at DTW, Oakland County Michigan had accessed my credit file (and my fathers) a couple times.
When I discovered this, I put a call into the police department there. The department pulling my report was their child support division. I have a cousin in the DTW area who is a deadbeat parent and from what I understand is still revolving in and out of jail for drug charges, not paying child support, etc.
While our names are completely different, I suspect this may have had something to do with it. I made a complaint with the police department and the Oakland Co court system for accessing my credit file. I also got them to remove the inquiries. The county said it was a "clerk error" that my TU file was pulled, my father got a similar excuse. Neither of us buy the excuses.
Finally one day the US Customs secondary searches ended and I haven't had to deal with it for a few years now, knock on wood. While I have no proof, I suspect my name somehow got referenced with his in some way - or they were on a data mining mission of some type.
After all my bad experiences at DTW, I do get a bit concerned when I have to clear US Customs and Immigrations there -- but thankfully I get through very quickly now. I still try to avoid clearing US Customs & Immigrations at DTW, but when I fly NW international, it is hard to avoid DTW.