Hello, I have gone through your query and understood the concern. If you are always regular with your contraceptive pill, your cycle is well protected and chances of conception are very poor. The occurrence of back ache and sticky white discharge calls for the suspicion of some infection; infections mostly delay or alter the menstrual cycle. Please get urinalysis, vaginal swab and a trans-vaginal sonogram to clarify the situation. You can also get a blood test for pregnancy to confirm/exclude pregnancy with more precision. Hope this helps.