Exercise During Periods

You can certainly do any workout you please during your periods. Just make sure you feel comfortable.

 

During periods, the uterus does move a little downward and the muscles do become more flexible.
If one is throwing 4 lb balls to wall and catching it or flipping big tires or any heavy-weight bearing exercise- then the likely hood of injury is very much increased.
The above is for someone who never had kids. After kids- the above kind of exercise will definitely hurt a woman during period.
In both cases- assumption is that the woman has not done similar weight bearing exercise as a teenager or young adult. And this thing is new for her.
If someone wants to start new exercise regime- one can start slow, be on your back and do weights, use lesser weights, know that most trainers are not aware of all possible injuries, do elliptical, walking, steps, bike, less weights.
All in all- your trainer was correct.

 

Contrary views in link:

Exercise During Period: What You Should Do and Avoid