An alternative hypothesis is that the reason you picked up that pack of gum in the first place, is that you'd already been served some ads for that gum in the past, but didn't consciously remember them (whereas your subconscious did, and said "yum" in that 7 eleven). Now that you've bought it you're consciously aware of the product and hence also notice its ads.

It would be cool to somehow verify this hypothesis. For example, you could request from Google/FB/etc to send you a list of all ads that were served to you in the past two months, and then comb through that and see if the pack of gum was in there somewhere.