Irregular Menstration

What if irregular menstruation mo? Masusunod pa din ba ung first day of the last mens? Aug 11 po kasi first day ng last mens ko, eh nagsex kmi ni bf october 11, pero sabi sa ultrasound ko 20 weeks and 4 days, meaning 4 months mahigit na na, e dapat 3 months lang. Ang gulo, paexplain po hehe