Should I Be Worried Of Being Pregnant When Periods Are Delayed?

Hi, My period is about 13 days late. I had sex about 23 days ago, protected and we're certain the condom didn't break. I've taken two home pregnancy tests too and both returned negative. I've had noises and movement in my stomach in the mornings, similar to what I get just before the start of my normal period, pretty much since the time my period was meant to start. My stomach kind of tightens and gets a bit tender after this. Just before the start of my last period I had a bladder infection and kidney stone and about 5 days before the expected start of my late one I was sick with some sort of tummy bug. I'm underweight, but have been my whole life and this has not previously affected my period. Could I be pregnant/what else could this be? Thanks so much!!