I used Monistat 3 for one day. Can I still go to the doc about a yeast infection?

It depends. Your physician would have difficulty diagnosing an infection if you have medication in your vagina. Unless you have other concerns to bring to your doctor's attention, you might want to just complete the treatment you have started and see if it is effective. If you still have symptoms, then you should be evaluated for yeast or another type of infection.