Some users have noted that their batteries have expanded and interfered with their trackpads. Is your MacBook one of those with the easily removeable battery? If so, shut it down, remove the battery and start up again (connected to a power source, obviously.) Then test the use of the trackpad. Does the behavior change?
Also examine the battery for signs of it expanding. If that is the case, replace it. Apple sells MacBook batteries for $129 (in the USA) and other users have reported being able to purchase 3rd party batteries for $20-30 from eBay.
Best of luck.