Experiencing shriveled buds/when do I repot?

[nailgun529]

Hello--

I've got a small collection of miniature "Phalaenopsis" (that's what it said on the tags, at least) orchids that I've had for about a month now. They've all come from our local grocery store, and I haven't had any problems with them. Within the last week, however, I've noticed shriveled buds, as well as half-opened buds and fully formed flowers that shrivel and fall off on their own on all but one of my orchids.

My question is, am I experiencing bud blast, or are my orchids dropping their blooms and need to be repotted? My plants are in an east-facing window, and get watered about twice a week.

Thanks!