My 13 day old baby boy hasn't had a poopie diaper since..?

Friday at noon. I'm concerned because before that day he didn't go for 2 days. He is being fed with the exception of 1-2 feedings where I supplemented with formula due to the fact I was so sore from feeding. Then that's when he didn't go for 2 days, I went back to milk then he went that Friday at noon, but that has been all since. I'm concerned why he isn't going. He has had plenty of wet diapers all day long and lots of gas but nothing more. What could be the problem? What should I do? He does have a Dr's appointment coming up so should I wait for the appointment or take him in or call about it? Any advice would be greatly appreciated. Thank you!