C.C.
This is the age when babies suddenly start pooping less. It happened with both of my daughters around that age. The vaccinations are probably a coincidence in timing. At around 2-3 months, babies' bodies become way more efficient at using food, and they simply stop pooping as much. I recall that it went from a several times per day event, to only going twice per week! If you're concerned, it's certainly worth a call to the doctor just to be sure, but just know that what you're experiencing is probably completely normal.