I've seen those symptoms from using the wrong bobbin. And since you said you just changed thread, be sure you're using the right bobbin for your machine. And if the bobbin is plastic, are there cuts and rough spots on the edge of the bobbin? There could also be a burr on the hook, or a pin under the plastic bobbin case. Are you comfortable with removing the needle plate and bobbin case? Looks like your machine is an easy one to do that with. Just un-screw the screws and lift the bobbin case out. Clean out any lint or debris, and put it back together. Pay attention to where the little bump is, it goes on the right, front of the bobbin area, and will bump up against a little spring. I have a picture on my blog, but am not allowed to put a link to it for some reason.
Good Luck!