I think that it can be searched with qualitative tools. The relation is that while having more burden will definitely enhance the stress level

My two cents. When designed with a proper pedagogical framework and implemented with the right instructional approach, I think a learning tool should never cause cognitive overload. If it does, perhaps some elements of the tool should be revisited. If we look at your case from the perspective of cognitive load theory, it seems to me that the students' limited working space (aka short term/working memory) is overloaded. In such case, usually they opt for surface learning, or do not learn at all. The headache might be caused by forcing the already full capacity with more information.

Cognition is the mental process by which new information is processed, transformed, stored and retrieved in our memory systems. Cognitive load refers to the amount of mental processing needed to solve a problem or to learn a new concept. The perceived cognitive load to a learner depends on the inherent complexity of the task and the way the information/task is presented to the learners. If these two aspects are less than the working memory capacity of the learner, they will not experience a cognitive load. Now, cognitive overload is a situation where the teacher gives too much of information or too many tasks simultaneously resulting the learner to struggle to process all the information.

Fatemah, in your given situation, it is important to identify if it is high cognitive load or cognitive overload. Further, it is also interesting to know the way you managed to measure the cognitive load. Higher cognitive load clearly hampers learning, but may not cause headache. On the other hand, cognitive overload could possibly result a headache.