Why am i feeling movement in my lower abdomen if im not pregnant

This may sound strange but I swear i feel something moving inside me in my lower abdomen/stomach. Ive been pregnant before so i know what it feels like when a baby moves and it sort of feels like that. ive taken pregnancy test and they've all came out negative. Back in August i was in the hospital with a kidney stone and ever since then my period schedule has been all over the place. sometimes ill get it two weeks early other times i dont get it at all and sometimes i get it two weeks early for a few days and then again for a few days a couple weeks later. is there a chance that i could be pregnant and it just not showing up on the pregnancy tests that i bought? i only tried one kind because it was cheaper. ive also been getting random sharp pains. What other explanation is there for feeling something moving inside me. or have i been worrying so much its all in my head? lol