I recently got offers from 2 labs to do a PhD and accepted one and rejected the other. I now don't know if I made the right choice. Would it be wrong to contact the other lab and see if there is still a place for me even though it would be behind the other lab's back? I am worried about how this might affect my future career in science since the 2 labs are in the same field.

I'm in a similar situation involving a job offer and a phd offer. My advice is this - at the end of the day the most important person in this situation is you. You're the one who has to live with your decisions. Ideally you don't want to piss off any potential employers/research groups but if that’s the price of doing the phd that’s right for you is that so bad? I would have thought doing a phd that’s not right for you is worse.

Science is a broad field and if you did upset this particular research group i'm sure there are plenty of other opportunities out there to compensate. Besides no one can really blame you for starting a phd and realising its not for you. No-one has 20x20 hindsight... except for maybe superman... and he wears his pants outside his trousers (which are made of lycra) so who is he to judge.