What Causes Pain In Lower Abdomen With Delayed Period?

hi, this is the first time ive missed my period, im having lower waist pains and some times get heat strokes. I've taken a pregancy test and it came out negative, im very emotional and some times have cramps in my stomache. I was pervious diagnosed with a STI and got it treated. Iv had my periods for the previous months and nothing was wrong. please shed some light