If your last period was 3 weeks ago, it would likely be too early for a pregnancy test to show anything, but if you're saying you used one 3 weeks ago, which was several weeks after your last period, and it confirmed ovulation and you've developed symptoms since then, then it's possible you could be pregnant if your period hasn't show up again yet (it's late). If it isn't late, it's possible it's too early to know. If you take a an opk now, what does it show? Is it positive (as in, test line as dark as or darker than the control line)? Luteinizing hormone and hcg are very molecularly similar, so often an opk can pick up pregnancy if you are around the time of your missed period or after. But you have to read it the same way you'd read an opk, as in the test like has to be as dark or darker than the control line, not just a faint second line. They often aren't as sensitive though as the early pregnancy tests, so if you've used a decent brand of pregnancy test (not one of these cheap ones), it should pick up pregnancy before an opk would.
With this pregnancy, I did take an opk at the same time I took a pregnancy test (16 dpo) just because I was curious. It was very positive, but so were my pregnancy tests. Do you know how many days post ovulation you are? That might give a better idea if you should take another test or if it's likely something other than pregnancy that's causing you to feeling like this.