According to Very Well Mind*, the right amount of stress can increase your memory performance. But too much stress inhibits learning because the stress hormones that are released impede the formation of memories. This is not helpful if you are trying to learn a new concept or recall an old concept for a test. Stress can also be exhausting which makes it difficult to learn. The good news is there are things you can do to reduce your stress. The Smiling Mind app has free guided meditations and mindful activities - some as short as three minutes that you can do before and during your study. 
* https://www.verywellmind.com/stress-and-your-memory-4158323

I used to be scared of asking questions at school in case I looked stupid, but then I would end up looking stupid because I hadn't asked the question. The worst case scenario is that your question is a silly one and someone might giggle or the teacher might grumble. On the other hand, if you ask a question, it shows that you are interested, listening and thinking about the topic. It is also better to ask a question and check that you only have to do questions 3 and 4 for homework, rather than staying silent and doing questions 1-10 because you weren't sure. 99% of the time it's better to ask the question.