Baader-Meinhof Phenomenon: you were having ad-blindness until your purchase that brought up to your conscious level the "Voltaren" concept. As you cannot recall the previous cases where you saw the ad because it did not make it into your conscious mind, it seems that the ads started only after your purchase.

Your perception works in 2 modes - bottom-up (signal processing) - and the ad didn't make it through - and top-down (pattern recognition) - exactly the experience you report - you recognise a recent pattern.