What Causes Nausea During A Menstrual Period?

hi.i underwent a surgical termination about 2 months ago on first week of dec.i got my period on jan 10 then after 10 days i had again have periods on 26th jan and not yet stopped.i have nausea this morning like when i had in pregnancy .i m also using contraceptic pills now.is it ok to if i have nausea with my periods?