Nicky is the only omega on a team full of alphas. When he and Joe were kidnapped, Dr Kozak injected Nicky with something just before Nile broke everyone out.

It's not until they get to a safehouse after that Nicky starts to go into heat. Normally, his system burns through heat in a matter of hours. He feels more horny, but it's soon over. But not this time. This time he's burning up, and after a couple of days, Joe's too exhausted to keep up. So he calls in reinforcements in the rest of the team.