Can Pills Cause Irregular Periods?

im 26weeks and 2 days pregnant now! based on ultrasound i take pills for 6 months and stop on November, My LMP was Oct.25. i had unprotected sex on Nov. 13 16 and 17. i didnt have my period on Nov. then December 6 i had protected sex with a friend, we used condom. But my ultrasound of off like 4 weeks from my LMP. coz im sure i didnt have my period on NOV and my LMP is Oct 25 to 29.. is this an effect of taking pills?? im confused!! is it possible that i ovulate late than expected? i dont know if i will count based on my LMP or my ultrasound. My ultrasound is always different.. 1st vaginal ultrasound. 3 weeks on Dec.27 2nd abdomen ultrasound. 7weeks on jan14 3rd abdomen ultrasound. 14 to 15 weeks. march 1 as i thought i was 12 weeks 4th 21 weeks on May21.. theres always diffenrence 1 to 2 weeks in these ultrasounds.. help please!! thanks