I have been having this problem recently too. I wear tampons even when im not on my period because discharge. But when i lay straight on my back it starts to hurt within minutes. And everytime i have a tampon in my lower stomach hurts and feels almost like cramps or something. Sometimes it hurts so bad i almost throw up.
That sounds like every symptom is from your period!!!! Have you started yours yet? If not, then yours will be starting very soon so don't be scared. That is all very normal, so be prepared and have pads or tampons on hand! Good luck, and you are absolutely NOT alone ;)