Opk AM testing

Liz

Newbie ttc month 1. I got a positive opk tonight after feeling pain on my right side. We BD'd and now I usually test daily when I get home from work. I leave by 6am and want to test in the morning as well as when I get home at 8pm to see if it's still peaked. What is the reasoning for not using first morning urine? Is it okay to test in the morning since I already got a positive? Thanks y'all!