"I'm shocked," Eftimova, who will turn 24 next month, told local channel bTV. "I still don't have an explanation because the championships in Daegu were held 10 months ago.
"I received e-mails from (world anti-doping agency) WADA and (world's athletics governing body) IAAF and I was informed that I could complicate the situation if I decide to appeal."
Eftimova, who competed at the Beijing Olympics in 2008, was expected to run in three events in London - the 100m, 200m and 4x100m relay.
"I was preparing for the Olympics and we're hoping to form a strong relay team," added Eftimova. "And suddenly ... I still don't know anything about my punishment but I'll not give up and I'll come back even stronger."
Bulgarian sport has been marred by a series of doping offences in the last few months.
In January, three CSKA Sofia football club players were suspended for three months after testing positive for methylhexanamine while tennis player Dimitar Kutrovsky was banned for two years earlier this week after a failed dope test for the same stimulant.