Is feeling nauseous before periods a normal thing?what causes it?

Yes: Unfortunately! the dominant Progesterone at the end of a cycle, just before a new cycle is also marked by an increase in prostaglandins which are neurotransmitters that affect our gastrointestinal tract. Some women have nausea, others have diarrhea. Lots feel bloated and gassy. Not fair, right? Bottom line is, it's completely normal. An non-steroidal anti-inflammatory like Ibuprofen is best.