You have sensed fear perhaps because you are not sure what happened. A
tow through booming lift (and the sink that goes with it hand in hand)
can make you fearful, but once you understand what happened some of
the fear will go away.

Talk to your instructor about how you need to be quick to respond to
the effects of flying through thermals on tow. Everything happens
faster and it is good practice. As you found out it means a great
flight is just ahead. Hopefully the upset feeling will diminish as you
get comfortable with stronger conditions and and experience lower
stress levels. I think it is normal to feel a bit upset if someone
else is flying on a bumpy day.