I took an antibiotic and didn't use a back-up about a month ago. I didn't get my period for that month (well I had one day so I'm not sure. And even that one day was about a week or so late). Does an antibiotic interfere with an IUD like they do the pill? It's getting kinda close to when my next cycle would be (two weeks.). My periods have been about a month and a half between cycles, and I'm cramping all of a sudden. For frame of reference, I've had my IUD since Feb '15. I've also never been able to find my strings. Any help with these wonderings would be wonderful.