7 weeks pregnant but apparently 5 weeks? Imminent miscarriage?

So my last cycle I miscarried at 4.5 weeks, Which was october 21st. I counted that date as my LMP, but maybe I was totally wrong? I got a light positive on November 16th on an FRER,but didn't start getting positive digitals until 2 weeks later. A week after my first positive I got a blood test which was negative with my HCG levels less than 5. How in the world did I get a positive FRER and then a negative blood test?Then a on friday December 5th, I got another blood test and my levels were at 838. The nurse said that would make me around 4-5 weeks pregnant. I am now wondering if I am going to have a miscarriage, and am really worried. Either somehow I ovulated almost 2 weeks later than I thought I did, or I am in the right timeline but I will miscarry? I have cramps on and off, but no bleeding,and the cramps are more dull. Has anyone had anything similar to this experience? Sorry if any of this is confusing!

Not sure how much this will help but when I got my positive on the FRER test I went to my docs and got a blood test (I was roughly 4.5weeks) the HCG levels were 18 and my doctor said that was barely a positive and that they should be much higher although I went on to have a daughter 8 months later!
Try not to stress, easier said than done I know, but only time will tell and unfortunately there is little you can do at the moment.
I also had a dull feeling in my lower stomach early in my pregnancy, I wouldn't have called it an ache or pain it was more just like a solid feeling in my tummy - if this is what your feeling then I'm pretty sure it's normal :)