Pregnancy symptoms but negative test

So I had my implant replaced about 2 months ago (this is my second implant) and had a period that lasted a little longer than usual not long after. However, I’ve not had a period since then; it’s about 2 weeks late now. I’ve been feeling nauseous on and off for the last 3 days, and am more tired than usual. I’ve taken a couple of pregnancy tests (including one at the doctor’s surgery when I was concerned at around the 11 day late mark) and they’ve come back negative, plus I’ve never had issues on the implant before, so I’m pretty sure I’m not pregnant. Any ideas as to why the missed period/nausea/fatigue? I’m not too concerned, just a bit confused! Thanks!