I think it's the end of the flowering cycle, if the leaves still look good. While you've only had the plant for 3-4 weeks, there's no telling how long the flowers had already been open.

My recommendation to you would be to put in a slightly brighter window (it can take morning sun, but get it used to more light gradually), give it warmth, continue watering only when it's dry and you can fertilize, if you'd like. Be sure to fertilize at a quarter of the recommended strength on the carton. I'll also draw your attention to a culture sheet produced by the Canadian Orchid Congress, which I find useful for those of us who are in non-tropical zones and grow inside.

Julie

Here is a link that might be useful: Canadian Orchid Congress - Phal. culture sheet

My recent experience on this is a little different from jamcm's.

While it is true that you may not know how long the flowers have been open, Phal blooms can last up to three months--sometimes more.

After noticing in previous blooming cycles that I would lose a bloom or two right after fertilizing, I have stopped all fertilizing while my Phals are in bloom.

In addition, two weeks ago, one of my Phals in bloom had three flowers wilt in succession. It was wayyy sooner than I would have normally tested it for needing water, but when I stuck my finger deep into the medium, it was bone dry! Normal watering (no fertilizer) stopped the wilt.

I suspect that this Phal (and possibly all Phals) may use water faster, when they are in bloom. Since I don't know this to be true, I'm currently checking more often and recording watering times to test this "theory."

Sweetcicely

I think it is true, Sweetcicely. Unfortunately, however, if a phal in bloom is underwatered and the blooms wilt, that's it, they're gone. They don't re-plump back up to any great degree. If you think about it, when a phal puts out a nice long spike with several nice sized blooms on it, it has in fact doubled or more its surface area. Surface area that is full of pores that lose moisture to the air in the (usually) less than stellar humidity of indoor environments.

H

Thanks, Howard. I just meant that it seemed intuitively evident that they would use more water when blooming. In the absence of direct proof and knowing that intuition can be wrong, I didn't want to state it as a "fact."

Watering stopped the wilt, as I said, but of course, it did not rehydrate the wilted blooms. I was sad to lose the three flowers, but happy to be able to remedy the cause and save the other 13.

The important point for a new grower is NOT to leave the impression that they should water a blooming Phal more often, but that they should Check More Often and water only when the medium is barely moist. Overwatering is still a threat to the health of the plant, whether it is blooming or not.

Sweetcicely

I've seen the same orchids for a month or two at my local Trader Joes. I suspect it could definitely be at the end of its flowering cycle. You never know how long they've been at the store. And if the plant itself is healthy (sounds like it with how you've been taking care of it), then it will grow and eventually flower again for you :)