Is the bobbin thread making small loops, or are there small loops on the bottom (back). If the loops are on the back, it usually means the upper tension is off. When threading you may be missing the tension loops, or not lowering your presser foot. When you thread, you normally are told to have your presser foot up, so there is no tension on the discs and your thread will easily pass between them, when you get all the way down to the needle, you put the presser foot down to apply tension and thread. If you are doing all this, you might have something caught in the tension discs. Put the presser foot up and try 'flossing' between them with some heavy thread.
A good way to tell if it's the upper or bobbin thread is use different colors in the top & bobbin.