Single tap on the trackpad highlights?

This has happened twice before, without changing any settings whatsoever (well, to my knowledge anyway). Usually, a single tap on the trackpad would register as a click, but instead of clicking, the cursor locks on and it highlights everything. The only way to properly click and select stuff on-screen was to press on the touchpad. This problem usually surfaces very suddenly. I was wondering if I had accidentally activated some shortcut... how I should rectify this? Thanks in advance:)