The midfielder hadn't been feeling right; suffering from fatigue early in matches, and experiencing dizzy spells during training. The 29-year-old was determined to get to the bottom of things, knowing it wasn't his pre-existing diabetes that was affecting his performance and health.
Test after test was undertaken until a heart scan at Hampden prompted the Hibs medical team to send him to London to meet with a specialist cardiologist.
"When I was sitting in the cardiologist's office, I'd be lying if I said most of me wasn't thinking this could be it," Allan told the Scottish Football Podcast on BBC Radio Scotland.
"But something in me just felt that it might not be over. When I got a second opinion and did the same tests and came through with flying colours, the first thing I asked the second cardiologist was, 'can I play again?'
"I got positive news and that allowed me to get back on the park. But it wasn't a case of the test going well and I could start training on Monday; I had to go through rigorous things to get me back involved – even just to see how I would cope with the demands of training."
Allan admits that having a young son was part of the reason behind his exhaustive regime.
"I didn't want to put my son in a position where, because I might be risking my health, that might affect his life. That's why we went through so many tests just to get back on the training ground.
"It was a six-week programme, probably the hardest pre-season I've done. But now I physically feel the best I've felt throughout my whole career."
The former Dundee United youngster is not alone in combining a football career with hypertrophic cardiomyopathy, and revealed that a conversation with Dutch player Daley Blind reassured him that he still had a future in the game.
Allan continued: "Daley Blind at Ajax has a more serious case than me and he plays with a machine under his skin.
"Last year he fainted twice on the park. He's got the same condition, and he's come back twice from that.
"He's played for the Netherlands, and in the Champions League with Ajax.
"I had a good conversation with him and I was reassured by that, and that's with my condition not being as bad as his."
For now though, Allan is desperate to help Hibs finish the season strongly. The current coronavirus situation, however, is holding things up.
"We've been trying to get games arranged for me just to get a full 90 minutes for the last two months," he explained.
"We can't get referees, can’t get linesmen, some clubs haven't been tested... I've just been getting all my fitness from training. You can train all you want but nothing prepares you for a competitive football game.
"But everything now is just about getting back on the park and being the player I know I can be, if not better.
"I just keep going back to how good I feel. I definitely believe I've got my best years ahead of me."