What Causes Nausea During Pregnancy?

Hi, I m 14 1/4 weeks pregnant with my second baby. This pregnancy I ve got off pretty lightly with nausea compared to last time (this time I just felt sick for around 1/2 hour a day from weeks 7-9, but then felt fine since. I do quite often still feel faint in the morning, but this usually goes away after having breakfast. I haven t vomited last pregnancy despite feeling much more sick. This pregnancy I hadn t either until today! I felt more faint than usual this morning. It seemed to improve after a couple of small chocolate squares. At 12:15 I started feeling a bit sick, and by 2 I vomited once. I ve felt really weak since but not sick. I ve eaten some dinner but just don t have an appetite as big as normal. I feel faint if I stand for too long. I m worried I have got something that could affect my unborn baby? Could it be listeriosis? I ve got no flu like symptoms though, just those described above, and I m super careful with food and hygiene at the moment. Does this sound like an illness or late morning sickness? Hopefully someone could help as I m worried and searching online is not helping.