You are extremely unlikely to get pregnant from what you described. Sperm can only swim; they can't walk. So they need fluid to swim in. They are very unlikely to swim through yoga pants. With open-weave underwear (like lace) there is a chance of swimming through in pre-cum, but hardly through yoga pants as well. Then sperm needs to swim into your vagina to get you pregnant. You would even then have to be very wet.
"I didn't touch myself down their until I came home and changed my underwear" so how did you touch yourself? Do you mean masturbation? Do you masturbate with your thumb? I would think even then that any precum would have been wiped off previously, and sperm only lives for an hour or so outside the body.
However you said this happened yesterday, and you have missed your period (today). Whatever you did yesterday would not impact on a period otherwise due today. What you did a fortnight earlier might have done.
Hope this helps