I had a similar problem. Have you got a computerised machine. Have you taken the bobbin case out to clean your machine? I took mine out and put it back wrong and then broke the needle because it punctured the lower case which holds the bobbin. Had to buy a new bobbin case, but that didnīt work because I had messed up the timing on the machine.
If you have two different weights of thread on your machine that could be a problem. Have you changed your needle? On computerised machines they get very fussy as to whether you have a sharp needle or not.
Rethread it totally, if none of that works, I dunno. Mine is having the timing reset because that is what is causing the lower thread to be really loose.
Hope some of that helps you. Elle