So confused when am i due

So confused I'm pretty sure I conceived may1st last period was April 11 (first day of AF) and I went to my Dr she said based on my last AF I'm due January 18th and I'm 6 weeks along I went to the ER due to cramping no bleeding they took ultrasound and said they only see a sac that I'm only 5 weeks and it could end up a miscarriage or be to early to tell.. I'm so confused if I conceived may 1st then when am I due?? Having all the symptoms still of pregnancy