Hi Bobsie
I'm sorry to hear about your loss.
I just wanted to say a couple of things...
It is normal to get a positive opk one day and a negative the next (and it does not mean you are not ovulating). This happens because OPKs detect your LH surge which is what triggers ovulation. Because what they are looking for is a high level of hormones that often does not last more than a few hours, you will often not get more than one positive OPK. Many of the OPK instructions suggest that you should stop testing after you get the first positive. It is average to ovulate within about 48 hours of the surge first being detected, but you might take longer to ovulate or you might not then ovulate at all (or you might have a second surge and ovulation later in the month)
Even if you do get a positive OPK this does not necessarily mean you are ovulating. The LH surge triggers ovulation, but you won't always ovulate after an LH surge. Sometimes the body gears up for ovulation and then it doesn't happen. This doesn't mean there is anything wrong - I think this may well happen to us all once in a while.
Also, the first cycle after a loss can be really all over the place. I found that I ovulated at least a week to two weeks later than normal after my losses. Annoying, isn't it?
As for the cramping, that could be pregnancy or it could be your period coming (or it could even be that you are ovulating later than you thought). In these circumstances it is probably best to aim to have regular sex (3-4 times per week) throughout your cycle. I don't think you should be overly concerned about the cramping - what I have found is that the first cycle after a loss can feel "different" and that I tend to be hyper-sensitive to every twinge.
Anyway, I hope that it is implantation and that you find yourself pg again very soon.