I randomly have a throb in my urethra during the day. I don't have pain or burning sensations when peeing and no urge to frequently urinate. What is it?

Probably not serious. Assuming you haven't had any discharge of pus or mucus fromt he penis, I would guess it's nothing more than genitally focused anxiety. But if in doubt, see a doctor. The Honolulu and Hawaii health dept STD clinics are very good. Check it out -- probably nothing serioius. Good luck!