Should I get this checked out, or wait to see if it heals on its own?

Today, I went to the gym, after not working out for about a week. I normally do two sets of squats (12 reps) with 3 lb weights then two sets of 12 lunges, also with weights. Today, I upped the weights to 5 lb, but only did one set for both lunges and squats. I also rode the bike for quite a while.

I was perfectly fine until now, when my right knee locked up. I can stand semi-straight, as long as my knee is slightly bent. Whenever I try to stand my tallest and straighten my knee all the way, it starts to hurt to the point where I can’t stand it. Also, when I sit with my knees bent, there’s a dull pain, but not enough for me to notice if I’m focused on something else. As this has never happened to me before, I wanted to know if I should wait a day or two to see if it works out on its own, or if I should go to the doctor. So, could it be serious?