Nexplanon

Ok so I’ve been on the implant for almost 13 months now and I haven’t had a period since mid September. I haven’t been sexually active since early August, and I took a few pregnancy tests up to 3 and a half weeks after that sexual activity. All negative. I’ve skipped before with the implant but never for this long and it’s freaking me out a little. I haven’t really noticed any pregnancy symptoms and I don’t look pregnant at all, but I’m still a bit worried. I feel like if I was, the tests would have been positive and I’d have some symptoms, or maybe even see body changes considering if I am I’d be about 5 months by now. But nothing I’ve noticed is abnormal. If anything I’ve lost weight and thats it. Any thoughts? Has this ever happened to anyone?