Why Do I Have Cramps In Back And Legs After Taking Calvepen?

Hi there, I recently completed a course of Calvepen 666mg, 4 tabs a day for 10 days. I was due my period 9 days ago. I have experienced the usual cramps & sore back & legs however, no period. It s not a possibility that I m pregnant but I m normally very regular. Could the antibiotic have effected my menstrual cycle? Thanks, Net