Then, there are the less-physical causes of post-sex nausea. In some cases, if you're nervous or anxious about having sex , it can cause butterflies in your stomach or nausea, Dr. Gupta says. "Usually this kind of nausea will subside once the pleasurable experience of sex sets in," she says. Although, if anxiety and sickness hit you most times when you have sex , then that could be your body's not-so-subtle way of telling you that there's something about the experience or person that's making you uncomfortable. In which case, it's worth it to explore your emotional reaction with a mental health professional who you trust, like a therapist or counsellor.