If you thought she was joking, you wouldn't have posted this. You must think that she's hinting that your sex life is not satisfying to her.
You should talk about it with her. You can't be expected to get hard and be a perfect sex god all the time- it takes two people to make a relationship work and to keep it hot. So don't feel guilty... But at the same time do what you can to keep it hot. If there's a problem with the sex, there is a reason... You aren't as attracted to her, you're feeling insecure, you want to sleep with other people, you think SHE wants to sleep with other people... that kind of thing.