Contraction?

2months on healing mode and breastfeeding, nung ika 1 month pinag family planning na kme ng OB ko, take pills ang pinili namen.. then nagkaron ng isang beses palng na contact with husband, contact na hnd naman sa loob OB says its fine if both are ready, on time taking pills naman pero just today feeling ko, ung feeling na magkakaroon. Bakit po Kaya ganun? Sakaling magkaregla normal lng nman kahit hnd pa tapos ung tintake na pills?