03 coupe won't start
 

So here's the thing. About a week I noticed my car would crank really slowly then start. So I went and bought a new battery, then it still cranked slow then it wouldn't start eventually. I took my starter off and got it tested but it was fine. Still I don't trust the tests because its not tested on a fly wheel that gives resistance on the starter. Any ideas? Thanks in advance.

Not sure if this is an obvious answer but maybe alternator?