There's nothing strange about that. A dedicated opponent can find that you have used a VPN.
When VPN providers advertise (unverifiably) that they don't log, that means FBI won't find anything if they seize the VPN provider's assets. That protects you from fishing expeditions.
But many devices leave trails when you connect and disconnect from VPNs. Browsing history is saved, which sounds like was the case in this situation. Search history is saved by search engines. So even if you clear your browsing history afterwards, Google or Bing still remember what you searched for.
Anyone monitoring your ISP can see that your IP connects to a VPN service and unless there are multiple hops, it's trivial for a dedicated, resourceful opponent to compare the VPN IP you connected to with the same IP being used at the same time somewhere else. It's not a very strong link by itself since other users may been using the same IP at the same time, but it becomes a part of other evidence against you.
Security and privacy neither begin nor end with a VPN. It's just one tool you can use in a larger set of tools.