What Causes Delays In Menstruation?

Hello. I recently had a uti and was put on a nitrofurantoin. I finished the pills on Sunday. I had a lot of symptoms with this antibiotic, including fatigue, loss of appetite, eye pain, and headache. The uti is gone, and the symptoms of the antibiotics have subsided, but I was supposed to get my period on Aug 20th. It has yet to come, although I had mild acne and tender breasts, as I typically do with menstruation. My lower abdomen feels heavy or full and I get dull pains on my left side. I took a urine pregnancy test yesterday, which came back negative. My temperature is 99.7, and my face is flushed.