Hi,
My Qubes login screen briefly changed to a red screen displaying a general security warning. This lasted for a few minutes.
Could you please let me know what might have caused this?
Thanks,
Most likely it was a web-browser displaying a spoofed security alert.
Next time you must make a careful note of the exact details, or take a photo.
Edit points to the slightly slower but more complete answer of @OvalZero 
Please provide further detail:
- What hardware are you using to run Qubes OS?
- Which BIOS are you using: coreboot, heads or generic? In the case of heads: Did you sign the boot files?
- Did you take a screenshot or note down any messages that were displayed?
Edith adds: @phceac was waaaaaaaaaaaaaaaay faster. 
It didn’t go well with the qubes os login.
An image of a mouse appeared, after I restarted it went away.
What hardware are you using to run Qubes OS? Lenovo Thinkpad T480.
Which BIOS are you using: coreboot, heads or generic? UEFI.
In the case of heads: Did you sign the boot files?
I don’t have heads.
Did you take a screenshot or note down any messages that were displayed? No it was very quickly.
Could have been the XFCE logo. Something like this?
Do you use XFCE?
The red color/tint could indicate an overheating GPU, a crashed graphics driver or a loose monitor cable. Could be anything.
No error messages, you remember? A single word?
Exactly that photo. There were no error messages—it just appeared on the login screen—and I thought a hacker had exploited a security vulnerability.
I didn’t even know that was installed on Qubes. No error messages, just the image. It really freaked me out.