Is it normal that I have trouble remembering my manic periods?

It is common. It is very common to have trouble recalling details when one is very ill with mania. The ability to attend or pay attention is impaired when one is manic. If you don't attend in the first place, it is not possible to recall later.
Yes. In a way, that is a self protective mechanism. We might not always want to remember all the gory details of our worst moments.