Or it could be that, when you search something, you also click through to some of the results, where FB/AdWords and other trackers are present.

Alternatively, there was a fallacy of human mind(forgot the name), where if you think of something, your brain will start focusing on the random occurrences of that things in all places. So, those ads were always there, your brain just started focusing suddenly on those as you were thinking of those things.

Note: Not affiliated with DDG.