Your question is a very common one and you'll be pleased to know there is nothing wrong with your memory, it's as good as it should be.
The issue you are encountering is a lack of belief in your own ability and not enough practice yet.
As you learn each topic, it is central to your focus. You learn, you understand and as you say, you can answer the questions. When you move to the next topic you follow the same process, which means the topic you learnt last week is having to now share your focus in a less important position.
Sharing focus adds a delay to recall while your mind sorts the information you need, but instead of waiting and believing in your ability, you are confusing the delay with I can't remember. This is causing you to panic and then waste your time re-visiting lessons. Give the delay time to work and you will remember most of what you have learnt. Where there are gaps, it just means you need more practice and that will come naturally as you progress through the course.
Trust yourself, you knew it last week, you'll know it this week and next. Keep moving on to the next lesson because the more you move on, the more practice you will get and the shorter the recall delay time becomes.
Everything we learn we remember and store without exception any blocks in recall is just panic and lack of practice, relax and let it flow.