Well, in the midst of a bipolar high, and perhaps also in a very deep low, one may be confused enough not to be properly recording in memory the things that were ahpenning, and may not paying much attention to events, either. But because there can be so many different causes of memory problems, the best bet is to see a good shrink and perhaps also a good local physician, for a full assessment, to see what, if anything, they can find.
And as ... wisely points out, some sedative tranquillizers can cause temporary malfunction of our memory systems, just like being very drunk can