What Is Wrong With My 2005 Mazda 6 s V6?

[Chuck B]

So i just recently (2 Weeks Ago) bought a 2005 3.0L V6 Mazda 6 with 65000 miles, Ive driven it about 250 miles and have had absolutely no problems what so ever, not even a hiccup and I filled the gas tank last night at the same place i always do. this morning I realized that my oil was a little low so, I headed to my local service center, figured I would just get the oil changed while I was there, so after driving on the highway about 3 miles going about 60mph, when i got on the off ramp, and realized my car was making a really weird sound not like i had ever heard before to me it sounded like a hyena not like a slipping belt , it was a loud squeak that's seemed to be in time with the engine that was at low rpm at idle, so after pulling into a parking lot and shutting it down i started it again and the squeak was gone but still running low rpm and sound like a quite diesel truck and had a small shake about the same as it does when any car starts up. after it ran for a few minutes of it running it stalled, now every time i start it it will stay running but sounds and feels the same, i am totally lost, anything would help thanks.