What Causes Irregular Menstruation With Nausea?

My last period was roughly 2 weeks ago however it was unusually light. More like spotting than an actual period. I have also been having occasional nausea which is often triggered by smells that normally would not bother me. I have used a home pregnancy test about a week ago, result being negative. My next period is not due for another 2 weeks and wondering when if I can try another test prior to that time. I should also mention that I have an IUD that has been in place for almost 5 years.