I'd take the bobbin case out and make sure there is no thread trapped in it. Also examine the hook basket and make sure there isn't any threads trapped in or under it.
You do know there are 2 screws on the bobbin case, 1 for tension, the other to hold the end of the curved tension plate to the basket. If you mix them up, and tighten the tension screw instead, you will have loads of bottom tension. Can you easily pull the bobbin thread through the the tension spring with the bobbin and bobbin case in your hand? If not, the problem is right there in you hand. If easy to pull, it's not the tension adjustment.
@Wizcrafts should be along soon to give you much better help than I can.