Could be a tummy bug. My guy had that a few times when he was tiny. My dr said that the difference in poo colour means the intestines are moving faster or slower. The mucus is an indication of a virus working it's way through. The same way we get extra snotty noses when we have a cold, the same thing happens in the gut.
Could be related to the injections in that it was a change to the system. I'd keep an eye on it for a couple days. My dr said to wait 4 days to see if it gets any better, if not then to come back. Every time, my guys never lasted more than 2-3 days.