What did it feel like?

Just wondering because I last tested at 12dpo, nevative, then got what I assume was my period at 15dpo but only lasted a day, and I had very light cramping. Fast forward to 5 days later I am so incredibly tired and nauseous. I highly doubt I’m pregnant but am going to take a test tonight just to rule it out. Just sitting here at work wondering if this has happened to anyone else!