The My Bids and My Watchlist are on the bidders tab for 20 days after the auction closes. After that time, they are no longer shown there.
When a bidder accesses their Watchlist/Bids, we look back -20 up to 'now' for items to display. This limits the results to a manageable number that will display quickly for the bidder.
If you want to check your won items, please take a look under "my invoice" as they stay there forever with your account.
Hope this helps!