Has my son got into a habit? If so how do I break it?

My son has slept through the night since 5/6 weeks old. Apart from the sleep regressions hes been brilliant. But for the past 3 weeks now every night I put him down he will wake exactly 20 minutes after. I dont understand why. Hes not hungry. His nappy is clean. I thought it might be a sleep regression but my mum said by the sounds of it hes now got into the habit of it. What do you think?