In the sense of, is this something most people experience, probably not. However, in the sense of, is there something seriously wrong with me, that's probably not true either. I wonder when it is that you're feeling sick to your stomach at the thought of sex. Many people, especially women, do not want sex at times when they're not feeling safe and connected to their partner. Being asked or pressured for sex when you're still upset can make you feel angry, resentful, and used, which might translate into physical feelings like nausea or a headache.
Many couples have conflicts over how often to have sex .
It's also very common for people who have experienced physical or sexual abuse to have mixed feelings about sexual activity. Once you have a sense of when these feelings occur, you can start working on dealing with them. In many cases, communicating with your partner can help. Or you may need to tell your partner that sex when you're having a really bad day, like failing a test or feeling insecure, is out of the question unless you initiate it. If the issue is too big for you to deal with alone, you might want to seek the help of a counselor, alone or with your partner.