labor on one side?

Im 27 weeks and the doctors are worried that i will have this baby quite early because of the problems i had with my last son and i only waited 6 months before getting pregnant again so my muscles didnt go back like they should have. They have been doing sonograms monthly to monitor my amniotic valve? i think. anyways i lost my mucous plug last week (which i lost early with my son and he was full term) and i having contractions but only in one side. it starts in my back and comes around my belly only on my left though, i mean i know what labor feels like but is it normal to be only on one side or am i feeling something else?