Late ovulation pushed back due date?

So long story but stick with me. My last period was around the 7th of April, I have super long cycles, and had some spotting on the 8th of May. Thought that was my period at first but it was just light spotting. Tested on the 15th and got my first (faint) positive. I went to a clinic and they went based on my last due date, which right now I would be 7 weeks pregnant. Went for my first ultrasound tonight and all they saw was a yolk sack and the nurse said I'm measuring around 4-5 weeks. That would make more sense to me because that would put me getting pregnant when I had the spotting, so that could be implantation bleeding. I'm just so worried that nothing is going to develop any further and I'll miscarry because I'm not as far along as I "should be". Anyone have experience with this that could shed some light on the situation?TIA