self dilation check

On your fourth pregnancy you tend to know your way around I started self checking dilation with my last pregnancy I'm 36 weeks and 5 days I've been having sometime of all contractions so I decided to check myself I understand the checking process and I know my way around my cervix what I cannot understand is why is it that I Felt babies head and felt the waters bag, but when the doctor checked she said absolutely nothing was going on my cervix was not open now after we left the office I checked again and I felt babies head again why is that I know it's babies head it doesn't take rocket science to find it. Any one ever go though something like that?