Is you account one that you started yourself from the start or did you buy one off from one of those sites that sell them?
When you say go a lot of places, how far are you going and how (what mode of transport are you using) to get there?
I have herd of people flying to other countries or catching bullet trains that play Pokemon Go while waiting at the start terminals and playing soon after arriving at new destination get flagged like GPS cheats because of the catch/time/distance difference.
I know people that bought turn key L30 accounts that have had that message pop up. They also found a heap of their Pokemon with Red Lines struck through them at the same time that message appeared.