This is typically caused by insecure code on your website in which attackers are able to use that insecure code to place various things into your site. I recommend having your developer further review any code to avoid this happening in the future. You will also want to ensure that no additional files were inserted into your site that can re-activate the hack as well. Your best option would be to restore from any backups you have been making, then look through all of the code to ensure that it is secure.
- I think my website has been hacked
- Recovering after a Hack
If you have any further questions, feel free to post them below. Thank you, -John-Paul