Period/ovulation/OPK test questions

If you have your period but -opk can you just count back 14 days from the start of your period to see if you ovulated? Does that work or no? I'm not getting my hopes up this month well get pregnant because I've only been off the pill for 2 weeks now so I may get my perio in two weeks or in another 4 weeks if I get it then at all. So is there a chance I could ovulate and not know it because you need to ovulate to have a period right and the time between is always 14 days right?