I would advise buying a pack of wondfro on Amazon to use in correlation with the OPK. You get a lot more for your money. It took me awhile to figure out when I ovulated so at least for the first two months I didn't feel bad about peeing on the stick. I tend to ovulate early so I start at 10 days (counting from the day of my first period) using the cheap sticks once a day until the control lines start to match, then I pull out the OPK and look for the smiley. That 24-48 hour period is when you are most fertile.

I agree with previous poster - most likely a BFP would show up, especially if you use early response preg test.

For at least your first month, I would start using the OPK about six days after you start your period and continue using it until you get a positive. I would also use the first response ovulation kit - it gives you a smiley face when you get your surge so there is really little room for human error in interpreting the results. After a few months when you know you get your surge (if you get it like clockwork like me), you can use the OPK less or not at all.

If your only unprotected sex was 3 weeks ago, then if you got pregnant from that it would show on a test by now (you'd technically be around 5 weeks). Sorry
It's possible you will still ovulate, or that you ovulated within the last 2 weeks in which case you will get your period soon.

My period is 6 days late. I took a pregnancy test this morning and it was negative. I am not sure when I ovulated last month but our only unprotected sex was feb 1st. I want to start using an opk. I have no idea when to start using it. Could my period still be coming? Did I just skip a month (never have before) or could I still be pregnant and it's too early to show up? I don't want to waste the expensive opk. I have some bloating and occasional cramps. When my period was due, I was experiencing major cramps, and slight fever. Figured I was going to get my period but nothing. Then I thought I was going to get sick, started to get a runny nose and headache but that went quickly too. Surely a test would be positive if I was 3 weeks, correct? If I don't get my period, will I not ovulate?