Confused..

Dj • Girlmom 💜 Sophia&Braelyn&Araya-Mae
Im having a hard time understanding what my due date really is. I no when my last period was (may 13th) which making my due date Feb 17th. At my ultrasound they said baby was measuring small and changed my due date to Feb 27th. I had an appointment with my prenatal njrse and when she measured she said i was measuring at 27 weeks so i dont no what to go by. Advice?