No, she remained sinless. This was a special application of grace that, as far as anybody seems to know, was afforded only to her. The exact contours of it are a mystery (in the Catholic use of the idiom) but, to draw a parallel, when a Christian goes to Heaven he will not sin. He will be purified from sin because he has faith in Our Lord.
Our Lady is no different in that respect. But where she is different is the same grace that other Christians receive after death she received at her conception. It's effectively the same grace; merely applied to her differently. Her sinlessness doesn't come from herself or her own merit. Rather, it still comes from God. Indeed, it can't come from anywhere else. The issue here is God simply gave it to her on this side of death.
I'm not sure if I'm explaining this very well because it's late and I'm getting a little punchy here. If not, I'll try clarifying this later on.