First of all, do check the link Alex posted above - this one here for clarity's sake https://marketplace.tf/blog/posts/YHLZOB - and take the suggested steps necessary to secure your account.
Second of all, you only mention trying to send an offer for this user's knife. Did you have any interaction with them, or anyone else regarding to this trade before it occurred? Alternatively, do you recall having logged into any new trading related sites in the past few days, or being asked to do so?
If either of those things are true, please provide uncropped (whole computer screen visible)/unedited screenshot proof of your interactions with all users involved, taken from here - https://help.steampowered.com/en/accountdata/GetFriendMessagesLog
When you take screenshots, please do so in a web browser rather than Steam client and hover your mouse over the name of the user you are interacting with so that their profile URL displays in the lower left hand corner of the screen. Then upload them to an image hosting service (imgur, for example) and post the links here.
Lastly, please provide a screenshot of your trade with the user that wound up with your item, taken from here https://steamcommunity.com/my/inventoryhistory/ and formatted in the same way - uncropped/unedited, taken in a web browser, and with your cursor hovering over their name so that their profile URL displays in the lower left.