To me it sounds like there's a chance it could be flat spots. Most folks will say 6 months, but a fellow who worked for a tire shop here told me it can happen in a month to 6 weeks depending on a few different factors. He said they tell folks if the tires are good quality and the problem is from sitting in storage or parked in the same place a long time to (1) overinflate the tires by 5 to 10 pounds, (2) drive the vehicle...preferably on warmer days and only for 50 to 60 miles at a time. He said not to drive faster than when the shaking is felt, not to take a long trip until you see if the situation improves (could cause odd tread wear or in some cases belt separation) and to expect it to take weeks to be okay unless you're doing this in the middle of the summer. I had this happen on a Pontiac I almost never drove. No amount of balancing, moving dots on tires to match the valve stem location, switching tires...nothing helped until the tires finally settled back down. Once they did, all was as it should have been. Good luck!
