Stream Entry is not lost. It is a direct experience and in some way alters us once and for all. I suppose you could forget the actual experience - through dementia or something like that - but short of something like alzheimer's I doubt you would forget it. According to the suttas, it is not something that is lost at death either (can't speak to that from personal experience though). As I experienced it, it would stand out as a very deep and profound moment or shift in your understanding/experience of who/what you are and how the sense of a separate self is created (and I mean really deep and really profound). Lots of words to describe something that is purely experiential.
If you are engaged in the noting practice that Daniel describes then I think there are other indicators for stream entry and someone else can describe those for you. It has not been my practice and I don't experience these things.
Daniel F Gurzynski:
I had many cases of feeling like floating, stretching, light, clarity, losing awareness and such.
If you can still call up these experiences there are ways of using them to develop insight and concentration.
-Chuck