You need to check your tension using whatever you're going to be sewing together before you start sewing in earnest. If there needs to be an adjustment, that would be the time to do it. I would think that your original tension adjustment is off if you have that big of a variation, even going from thick to thin.
The thread being straight along the bottom means the bobbin thread tension is tighter than the top thread tension. If you lifted the foot with the knee lift or the lever before you did your sewing, you have to make sure you get the top thread back down to the center of the tension disks, 'cuz lifting the foot releases those disks. If you start sewing before the thread re-seats itself that could cause the problem.
No one lives forever, the trick is creating something that will.