just got a new bike last week, havent been on it for more than 15 miles give or take and am changing something with the back wheel, i put the bolts back on and tighten then i spin it(the bike is on the saddle and handlebars on the floor) and the chain tightens, then slacks,then tightens each rotation by a GOOD amount, what is causing this and how the hell doifix it? its a brand new chain, thank you.