What Causes A Delayed Period And Lower Abdominal Pain?

i think i might be pregnant i have been on keflex 500 mg antibiotics i just finished them this morning i had a dose of 30 one could this harm the baby if i am and also i recently developed a case of vaginal thrush about 4 days before i finished my dose. im having some lower stomach pains when i stand of lay on my stomach but no blood i was due for my period at the begining of this month now here it is the 30th and still nothing