do i need a new bike chain?

[Tracey]

i have a schwinn sidewinder 2.6 fs and i got a new back wheel and new brakes two weeks ago. a week ago while i was riding, i had it on the 7 gear and the chain started to slip. so i changed it to the 6 gear and it did the same thing. so i changed all the gears around, and it did the same thing. i just thought that the chain was too dry so i put motor oil on the cassette and the chain. i was looking at the chain and it was kinda hanging. so is the chain old or too loose or too long? please tell me.