I'd get in touch with LastPass support asap to see if they have a digital trail to help you figure out what happened.
I'd also guess the most plausible situation would be malware on your computer that managed to sniff your credentials in-transit/clipboard/memory/browser/keyboard and exfiltrate it to some shady folks.
I'd also guess the most plausible situation would be malware on your computer that managed to sniff your credentials in-transit/clipboard/memory/browser/keyboard and exfiltrate it to some shady folks.