Pregnant or returning period

I’m 5 months postpartum and on the pill (the one you take while breastfeeding). My husband and I have been pretty sexually active. Two days ago I was nauseous and had a headache and then I was fine. Today I woke up nauseous until I ate breakfast. I’m going to take a pregnancy test tomorrow morning but I’m kinda freaking out. We have 3 and that’s all we want- like we’re in the process of getting the paperwork filled out for a vasectomy. But I also think this could be my period returning- but I can’t remember what the last times were like- any insight?