Period-like cramping when I sit or lie down, but goes when I stand?

Me and my partner are TTC, but my periods are sometimes irregular although it's mostly roughly a 34 day cycle meaning my period isn't due for a while seeing as my last one was the 7th of June. However I am getting period like cramps and backache which only occur when I sit or lie down. Do you think it could be just an early period or something else? I am also having heartburn.