Trying OPKs for the First Time

Anyone have any advice on how to read these? My app says I'm about to ovulate and it's high but how does it go from low to high overnight?(am I just being dumb?)

Trying to conceive after first miscarriage two months ago. Doc says most fertile within 3 months of miscarriage but just don't want to screw up my chances. Did the deed last night haha but of course hubby not feeling it tonight -___-.

Any help is much appreciated!