It’s a common problem with ageing Macbooks. In fact, every single Macbook I’ve ever owned has had this problem, some more than once.
It starts out with what seems like a bit of a bulge under the front of your Macbook. Soon the trackpad starts to have problems. Sometimes it seems like you can only press it when you apply pressure at the correct angles. Soon that bulge gets bigger and you can’t press the trackpad at all.