Upper left stomach pain..help

Tarah

What can cause cramps and pain right there. It goes into my back and will go sharp all of a sudden. It came out of nowhere very painful. I'm late on my period so I didn't have a frer so I did an opk to just see lol and it was faint positive but pregnancy test is negative. I ovulated the 4th. TMI I've been really constipated before and the last week all ive done is poop, so I know it's not that. Thanks