To expand on what Paris Geek has asked, - I suspect that your "Sent" folder in Aquamail has lost link to the corresponding folder on the server. This has been reported recently by a few Gmail users, when the default language in Gmail had jumped, and the name of "Sent" changed to a different language.
Long press on the account in the main window, choose “Options and folders”, then “Folders”. Or choose “Manage folders…”
See if Sent folder on the server is configured as "sync as [Sent]".