Bluestorm summed it up pretty well - since she got injured in Marbella she's been playing through pain pretty much ever since. We don't know for sure how she injured her groin, but we're guessing it was probably through overcompensating for her hamstring on her other leg which was injured a few weeks previously and maybe didn't fully heal. As i've said before in the livescore thread I think the reasons she is still playing on through injury rightly or wrongly are:
1) Because she doesn't want to get fined by skipping her commitment tournies.
2) Doesn't want to get 0 pointers stuck on her ranking.
3) Doesn't want to miss out on the $1m bonus for fullfilling all her commitments in a season - which is a lot of money to a young player like her who's just finished the season in the top 10 for the first time in her career.
Basically the WTA rules give their top 10 players incentives to play whilst not 100% fit which is crazy for any top level athletes
If you remember last season she pulled out of 4 events with only minor injuries, so i'm pretty sure it's the rules for top 10 players that are more or less forcing her to play. Whilst i would rather she hadn't played at all since Marbella i can kind of understand why she's playing bearing in mind all of the above reasons. The WTA shouldn't be putting their top players in this position though in the first place