Welcome to the EMD Forums, AlvinT!
There are several sub-forums here at EMD, and such Fastmail issues really belong in the
Fastmail Help and Current Issues Forum. I'm sure a moderator will move it there -- do not double-post, please.
I agree that a problem with the POP client in your Android smartphone is probably causing this. POP clients behaving badly can clear your messages on the server, since that is the default behavior of the POP standard (pull down all messages and delete them from the server). Fastmail recommends use of IMAP for this and other reasons. If for some reason you can't use IMAP on your smartphone, you can probably get it set up to "keep messages on server". But IMAP seamlessly keeps this working much more reliably. I don't have an Android phone so I can't give you detailed setup details, and so far there isn't any help for such devices in the
Fastmail online help or
Fastmail Wiki. But another user might provide some more specific assistance.
Bill