Jumpy/erratic Trackpad, PROBLEM

I have a white Macbook for about 4 years now, the keyboard stopped working and it never fell, it went to an apple store here in Portugal and they fixed it and changed the keyboard. Right after recieving it the trackpad was all jumpy and erratic. I really don't know if this has to do about when they changed the keyboard if they didn't hook up something right, I don't really know. It's been since october and I don't really have the money to fix this again, anyone know how much it costs or what I should do? Thank you!