Your account has probably been compromised. To be on the safe side, take the following steps:

1. Scan for malware https://www.malwarebytes.com/
2. Check that the email and phone number on the Steam account are still yours.
3. Deauthorize all other devices https://psteamproxy.yuanyoumao.com/twofactor/manage
4. Change passwords from a trusted/clean device.
5. Generate new backup codes for your mobile app https://psteamproxy.yuanyoumao.com/twofactor/manage
6. Revoke the API key https://psteamcommunity.yuanyoumao.com/dev/apikey