I had sex on the first day of my cycle and it stopped and never continued even days later. I'm on the nexplonon birth control but it has been expired since December of last year. It has been only a week since I had sex but I'm nauseous off and on and I'm wondering if it's just my mind messing with me or could I be pregnant. Any other time my cycle would have continued after having sex so I'm a little worried. Could I be pregnant?

The simple answer is yes, you could be pregnant. You're not on any useful birth control and you had sex. It also be your body adapting to not having the hormones supplied from your birth control implant. I would wait for about two weeks from when your period kind of started and then stopped and take a pregnancy test. If negative, wait until your next period should have come and see if you miss it again. My miss I mean by two weeks.

I would also recommend setting up an appointment to get your birth control renewed, provided you're not pregnant.

Just to add to what CravenMorhead has mentioned... it is wise to have it removed as there is the potential for problems if left after it has expired. Either have a new implant inserted or switch to something else.