A blood test
and a sonogram have ruled out pregnancy
and these are confirmatory. If you are still suspicious, you may ask your doctor to take serial titres of serum beta-hCG
along with serial sonograms to see if there is any obvious change. If you are pregnant, it would be wise to avoid antibiotics at an early stage. The cramps could be premenstrual. You may need to be investigated for the delayed period, if it is not a pregnancy; suitable treatment would put your system back on track and conception is made easy. Good luck.