I had this problem with my (sadly now defunct, as of a month ago!) 2011 MBP after years of use. You can try:
1. Resetting the System Management Controller. Shut down your Mac, be sure the power cable is plugged in, then press (left side) Shift-Control-Option and power-on keys all at the same time. Hold for about 5-10 secs, and reboot if it doesn't automatically (I can't recall if it does).
2. Reset the the PRAM/NVRAM. Shut down your Mac. Push the power-on key and hold down Command-Option and the "P" and "R" keys and hold them while the machine boots.
3. It may be a battery issue. The older batteries used to swell up when they got tired, and it may be pushing on the back of your trackpad.
hope this helps --
brad