So let me see. You say this started happening at level 39.
Lets take a look at ' Guarded by the Force'
Trainable at level 38.
Reduces the damage you take by 99% for 4 secs.
When the effect ends, 50% of your current health is lost.
So you are using 'Guarded by the Force' and when the effect ends bang goes half your health.
Hope this helps.